I'm not sure it's fair to assume a hard-and-fast rule here. While any child can be sleepy or cranky on any given flight, I think parents bear a responsibility to know their offspring...

If your kid is mild-mannered, somewhat self-entertaining, and generally easy to calm, I don't have an issue with it. If your kid is a screaming hell-spawn whom you can't control for 30 minutes at a time while you're on the ground, well...

I don't blame the child, I blame the parents when their kid is disrupting an entire flight. I know it's not always 100% controllable, but if you _know_ your kid is going to be a significant disruption to everyone around you, maybe F-cabin on a redeye is something you could be considerate enough to spare everyone from.