My research says that regardless of the width of the seat you'll never sleep at all with a screaming kid.

In all seriousness my Air NZ 77W economy flight back from LAX a few weeks ago was a horrible experience. The back of the plane was like a zoo full of screaming kids and the world's most uncomfortable economy seat I've ever come across. I'd had better 1-2 hour sleeps on an A320 to Aussie.