I think a big part of it is the child themselves. My ds needs sleep, I mean needs it. If he doesn't have it he is a whingy, grumpy child. Some kids don't need as much. Not that this helps your exhaustion. But I think a lot of the time there is only so much you can do.

Mind you I think my SIL encouraged bad sleep, because whenever her children would whimper she would tend to them. Hence waking them further, resulting in overtired children. Plus she would totally ignore tired cues. It's not about CIO, but it is about understanding your babies cries and cues. And this can go a long way in helping.

Just my opinion.