It can be done, that said, it can't be done in most startups.

Many startups are composed of mid-twenties white males. The culture of work becomes 10am to 8pm, then go drinking.

If you have a family, you could work 7am to 3:30pm and be out the door in time to see your family, then work some more at night. But you'll get glares, and eventually you won't be a "team player" or whatever they decide to make up.

If you can find a startup run by people with children it can be a good fit, but you don't know until you interview.