As an entrepreneur with two young children, I know it's hard to keep a balance.
The hard part isn't necessarily the limited number of hours. It's possible to build a successful startup while working 40 hours a week, provided you're really focused and not goofing around during those hours.
What's hard is many startups have a youth culture that glorifies in working all the time, though much of that time is drinking beer and playing foosball. That's great when you're 23. But I (and many others with kids) would much prefer a professional culture of focused, intense work from 10 AM to 6 PM.