I generally never had any problem with children on airplanes (I fell in that category not too many years ago), the problem, if they cause troubles, is the parents IMO.
The only occasion I have to complain about was two months ago, a CE service back to Milan, where the lovely child (around 8 years old) seated behind me decided to play with my hairs. I politely asked the mother to tell him to stop it and she incredibly said: "I won't say anything, he only wants to have some fun". Again, the parents are the problem in my opinion...