Since the English language does not have well-established rules about using gender-neutral pronouns, I always feel uncomfortable when I answer questions where the child's gender is unclear.
Is this OK to refer to a child as "he" in an answer, if the question is ambiguous about that?
Historically, "he" was generally used in the absence of a gender-neutral pronoun. But in a time of increasing political correctness, he can come across as presumptuous. The pronoun "it" is genderless, but using it to refer to people would likely be considered offensive. "One" is also used, but in conversational English, it sounds a bit silted and formal.
If you cannot reword the response to avoid playing the pronoun game, use the word "they".
From English Language & Usage: Gender neutral pronoun:
Singular they enjoys a long history of usage in English and can be used here: "Each student should save their questions until the end.". However "singular they" also enjoys a long history of criticism. If you are anxious about being criticized (for what is in fact a perfectly grammatical construction) you are advised to reword to avoid having to use a gender neutral singular third-person pronoun.
I often relate my answer to my own son and then it's easy to phrase it so that I can use his gender. Problem solved :-)
It's 100% okay by me, but some people would be deeply offended.
I've been using he/she and them in such questions. Neither sounds great, but it avoids the politically correct crowd throwing a fit. (Though I'm with you, if the gender is unknown, just picking one at random should be fine.)
