How well/badly the children behave is directly linked to how well/badly the parents behave. And I've seen some shocking parent's in the front cabins ...
The last time I saw bad behaviour was two girls about 7 + 9. They were in Premium Economy - 2 in front and 2 behind, each sitting with a parent. The girls started fighting over the back of the seat with each other and the parents took some time to stop it.
The most amusing thing I saw ... Again I was in PE and in the front row of PE on the Port side (just in front of me) were two children about 10/12 a boy and a girl. Their parents had the back two seats in Upper and it seemed like a good idea, so that the children would not feel 'stuck' with their parents and (presumably) kept the costs down and they were within 2m of each other.
It was a daylight CPT ~ LHR and by half way through - the children were in the Upper seats and the parents snoozing the PE seats!!! However, the children were very well behaved.