It probably depends quite a bit on the people starting the company. Our CEO, myself (CTO), and one of our developers are all fathers with children under 3 years old at home. A woman on our sales/support team recently took a chunk of time to have a baby, and has returned to working initially very few, now slowly-increasing hours.
We all work remotely, and sometimes at rather odd hours, to make this balance possible. It's not always easy, of course, but we figure it out.
If you're trying to actually have a presence in your kids' lives but no one else at work seems to have that facet in their lives, you're on your own, and your work (and peace of mind!) are going to suffer as you try to pass for a non-parent. But if you're working for people who know what you're dealing with, they can offer flexibility that non-parenting employees wouldn't need, share strategies and options, and trade stories.
This week, I've interviewed two candidates for senior developer roles who both have small children at home; I'm actually not sure if they were specifically interviewing with us because we're getting a rep for being parent-supportive, but that'd be interesting.
I can remember the other side of this; long before I ever thought about having kids, I remember when a fellow developer had his first child. He only took off for a week or so (we had 10 days vacation a year, and he stuck to that!), but then he would stumble into the office looking pale and his work went to hell. And of course his wife was home with the baby by herself all day, while he was at the office being mostly useless.
It didn't occur to me at the time that there might be a better way to handle it; though of course at the time it wasn't up to me anyway.