It depends on how you develop, really.
People in startups whose development is coding crazily 18 hours a day, obviously won't fit well with parenting. Of course, i'd be very impressed if you are coding 18 hours a day and aren't spending hours of that time rewriting code from early in the day/the day before (because it turned out to be broken or not well thought out or whatever).
When i was younger, i would hack at stuff very quickly until it worked (IE thousands of lines of code a day, it turns out,), and be proud of the result.
3 years later i'd look at the code and wonder what the hell was wrong with me.
Now i write code, if i hit a hard problem or things aren't working like i think they should, maybe i'll go take a walk for an hour to think about the problem and the code, then come back and code some more. I still end up getting more done at the end of the day, and the code looks better.
The extra time you get doing this kind of thing is the time you spend parenting.
You also get a much more acute sense of your limits as you get older, and know when you need to say "i just won't have time to do that, someone else needs to". Just because it's a startup doesn't mean people should try to take on every task. It doesn't help anyone if you say you can do something you can't :)