I don't think you should compliment on the kid even if he/she is very well behave for the entire flight...
Like I said before... many parents have snapped on me when I tried to talk to their children... Like I have to go through them before I could say anything to the kid...
I am not saying all parents are like this, but most are quite annoying actually... I always have to watch the parents before I say anything. If he or she seem cool...I would talk, if not, oh well...
Just DONT have kid , people. They are too much works, and very very expensive. Go adopt a pet instead okay ??