You got it. Sometimes you book late and don't have the luxury of seats together and then sometimes you DO have to rely on the kindness of others. We had to go to a funeral once and there were four seats on our flight but no 2 together which doesn't really work with kids under 5 but over 2.

Usually someone is generous, sometimes people are assholes about it, but never would I say "yo, give my kid your window seat".