How long is one obligated to keep a kid’s gift?

It's a gift. If they don't like it I hope they find a good destination for it. Maybe somebody else will be happy with it.

I give it away, and then I don't really care anymore, unless the gift was expensive and specifically asked for.

I don't waste my time by being offended about things that I have no control over. I've added my positive contribution to the situation, do with it what you want.