I blame the parents!!! LOL! DUH!
Unfortunately I think the parents fuel it by getting the kids all hyperactive about the thought of a plane ride and then while going through all the stress of the airport chuck loads of sweets and other sugary cr@p at them as a bribe to not go awol in the terminal. Then when they get on the aircraft they are like a loaded gun waiting to go off!!! Sadly a lot of parents then assume the opinion that they have paid for all the seats so the kids are now the cabin crews problem and not theirs. Heaven forbid a suffering crew member risk asking the parents to control their children, the gloves will really come off!