I'll answer why I thought that question was probably okay to stay open, and thus answered it. I'll cover the Good Subjective question checklist. Finally, I'll also explain why I answered it specifically. Apologies for the length in advance!
Why it is a good question for the site
First off, I fall much further on the "subjective" side of questions being acceptable than I think the average current member does. Some of that is a holdover from when I started - I feel like we had a lot more questions of this sort, and it was more acceptable with the community - and so I tend to be more tolerant of them then perhaps the community at large is. I'm aware of that, and so I won't vote to reopen questions of this nature if they're closed by the community - that's not what a moderator's vote is for. (I might cast a fifth vote, though, if it got 4 others.)
My feeling on questions is that, for Parenting, questions that can be answered by explaining how one might make a decision are acceptable. "Make this decision for me," and "What do you think, A or B?" questions, are not; they're just opinion, and the kind of thing a discussion forum is great for, not a Q&A forum.
However, questions that allow an answerer to elaborate on the elements of a decision, so, "How can I decide between A and B," are great. Those aren't opinion based: they're trying to get at the root of the question, and while there may not be a single correct answer, there is at least the potential for something that is generally helpful and useful there.
I think this is pretty much the case on any Stack - even Stack Overflow would probably tolerate questions of this nature to some extent, if there weren't already specific stacks (Software Engineers, Code Review, etc.) that are better fits for that kind of question in their various niches. Certainly PF&Money, The Workplace, etc. answer those questions all the time.
Now, where Parenting gets a little more squishy is the personal anecdotes side of things. We are different from other sites, in that personal anecdotes aren't just permitted, but encouraged, on some kinds of questions. From the help center "on topic" article, you have:
Please note that opinions shared here should be backed up either with a reference, or experiences that happened to you personally.
That latter part is important on Parenting: we recognize that not all questions can be answered with absolute facts, but can still be valuable questions.
The important part is that the question still be answerable in a meaningful way - not purely asking for opinion - and answers be formulated in a focused manner that does respect this difference. Providing personal experience is useful for some questions, because it allows one to see how a person made their decision - and thus take those factors into account in making their own decision. Anongoodnurse's answer to that question was definitely in that mold: it wasn't fact-based at all, but it effectively conveyed how she came to her decision when considering the same detail.
Thus, I feel like that question could be considered on topic: it asks how to make the decision, and asks for experiences as to how others made that decision. From the question:
I am seeking experiences from parents out there who have been in a similar situation, and what was, if anything, the thing that helped them decide whether or not to have more children.
That wasn't exactly how OP presented it initially, but that's what a good edit is for; and it definitely caught the spirit of OP's desires.
Now, let's check over the Good Subjective checklist.
First thing: before we do that, I think it's important to understand that on "how do I make the decision" questions, it's not critical to have a single answer to them, or "build consensus" as to the way to make the decision. It's also important to recognize what is the goal: it's to provide an approach to solving the problem. We're not trying to find the single element that is most important. If you want to look at what the consensus is building around, it's around the approach to the problem. In this case, the problem is "how do I decide whether to have another child or not", and the answers will each provide a potential approach; it is the community's job to rate how useful they are.
You'll note that Good Subjective not only does not look for a tight question with short answers, either: it in fact looks for long answers. We're not looking for just one of these things - we're looking for the holistic approach, because that gives you better answers. Each element - "does having 2 children mean they will be less lonely," "does having two children and not having enough attention for each cause a problem," etc. - is a poor subjective question because it's too much just opinion - some of them might be subject to evidence-based answers, but a lot of them are pretty squishy still even then.
Instead of having questions for each element, bringing them all together means that the answers are more useful and less opinion-based. Instead of giving your opinion on each element, you just show how you used those elements to make your decision - and that helps someone else see how they could make theirs, if they agree with your opinions (or even if not!).
Here's the rundown:
1. Great subjective questions inspire answers that explain “why” and “how”.
The question covers this: "why" did you decide on having 1 or 2 children, and "how" did you come do that decision?
2. Great subjective questions tend to have long, not short, answers.
Check - this definitely inspires longer answers, and is not just asking about one element: it's asking about the decision as a whole.
3. Great subjective questions have a constructive, fair, and impartial tone.
I think it qualifies here, also. The asker isn't asking a one-sided question: they legitimately don't know the answer, and explain the elements of both sides that make them unsure. I can't tell which side of the line the asker is on, and I feel like that's a good thing here.
4. Great subjective questions invite sharing experiences over opinions.
Post-edits, I think this definitely does this: it specifically calls out a desire for experiences, and doesn't ask for opinions on specific pieces of it. It's a very tight question (even before the edits, it was pretty tight) and doesn't meander about giving lots of room for opinions here and there; it lays out the issues the person has, and the specific things they think are relevant to the decision, and invites experiential answers that focus on showing how others made the same decision.
5. Great subjective questions insist that opinion be backed up with facts and references.
The question doesn't exactly do this, but it does very specifically ask for the experiences, and not for what the right decision is. I feel like it at least mostly meets this criterion.
6. Great subjective questions are more than just mindless social fun.
Definitely meets this: it's a meaningful problem that a person faces in real life.
Given that the question clearly meets at least 5, if not all 6, of the Great Subjective elements, and on that 6th at least passes by (or, if you are hesitant there, could easily be edited to meet this), I feel that the question fulfills Robert's goals here.
Now the last part: why I answered it how I did, and where this question can go wrong.
Questions asking how to make a decision are very risky questions, especially when they become HNQs, because they tend to attract poor answers that don't answer the 'how' but offer to make the decision instead. As such, it's vitally important for the first few answers to be high quality: you have a high quality example or two, at the top of the list, and you end up with somewhat better answers - or at least the good ones are at the top, and so the worse answers don't matter as much (as far as making it a useful question, and making it a good example of what is on the site).
That's why I answered how and when I did: I saw a question with the potential to go awry, with one good answer already, but the potential for a more evidence-based answer as well to hopefully bring in better answers in that vein (along with the already excellent purely experiential answer).
Did this question end up with bad answers? Maybe; one or two at the bottom aren't really ideal. But what I don't see are much in the way of "pure opinion" answers. One answer I feel like borders on this, and is definitely a poor answer; I probably would be justified in deleting it, but I tend to run with the soft touch approach to moderation, and prefer to let the community simply downvote content that's not good, unless it's clearly bad.
What the question did end up with were several good answers, some more factual-based some less, but all solid experiences. Beyond the two first answers from the moderation team, we have several experiential based answers that explain why they did or did not have a second child, one that explains how they felt after they had their second by accident, and two that give different points of view on the more technical (financial/time) elements. Are they perfect? No; but I think they add value in this format, and allow us to get what we need out of the question.
One last note: I think it's important to have questions like this, in part because it allows us to build a greater sense of community than the straightforward, short-answer questions. Yes, this isn't exactly the Stack Exchange model; but it also is what works for parents. We like to tell people about our kids, and we like to tell people what they should do! We avoid a lot of that on this site, in part for good reasons (this isn't a discussion forum), but it also loses us something I feel. This kind of question, while not perfect in some ways, does help us build that sense of community that we otherwise lack: it lets people provide those experiences.
This is something that Parenting has faced over its years: how do we balance that need for community-building with the need for ideal, perfect fit questions for the StackExchange model? The number of engaged participants in our community is very low right now, and while some of that is probably for other reasons, some of it is due to the difficulty in feeling like a part of the community, in my mind. SE gets a reputation of being unwelcoming, in part because of limiting the questions too aggressively sometimes. Maybe it's time to open things up a little - not to bad questions, but to ones that are a little less perfect fits, and not let the perfect be the enemy of the good?