I can always tell (by the look in their faces) who is a parent and who is not. Whenever I encounter a child who is not behaving (within norms) and the parents are doing nothing about it, I take it up with the child directly. I can put on a pretty mean, scary face and it works every time.

If an airline does no provide a changing tray on board, I see no alternative but to change the baby on the seat. If you have lots of practice you can get a poopy diaper off and properly taken care very quickly.