Migration is intended to happen when a question is off topic for the site on which it was first asked, but it could be salvaged by being posted on a different site. Encouraging a child in an activity is completely on topic for parenting. Just because you believe to could get better answers elsewhere (which, incidentally, is a gamble) is not a reason to migrate; that's true whether it is your question or another user's question.
- As of today, four answers provide a variety of advice, and none had downvotes. One is fairly comprehensive and provides numerous sugestions to encourage a child. (While I believe there may have been only three when you posted your meta question, that is still a number that indicates a substantial amount of attention, interest, and capacity to answer in our own community.)
- You mention "postive comments" -- I did delete one which was barely on-topic and largely talked about avoiding bikes entirely, which isn't very encouraging. I also commented on one answer which said little more than balance bikes aren't useful. Note that a better approach if there are tone or content problems is to flag individual answers or comments for moderator attention, not migrate the question. If you'd flagged content, a moderator could have helped with that much earlier.
- Since it isn't your question, you can ask your own version on Bicycles instead. (Indeed, you're already an active user over there, so that is even easier.) A link back to this original question which inspired your interest in the topic will help them to understand the background, and you should also be specific about what you feel is lacking in the current version.
- Posting a bounty is a great way to help attract more interest. Quality and variety of answers benefit you, the OP, the Parenting site in general, and anybody in future who does a Google search for encouraging a toddler to ride a balance bike.
I don't disagree with the decision to decline migration and would have done the same myself had I handled the flag. There are many ways to deal with what you perceive as inadequate response from the community that leave the question where it is.
