I've worked in two early stage start-ups since I became a dad a few years ago, as employee 1 and 2. I do a few things to help me stay involved as a father:

1. Unless there's a real emergency, which is rare, I leave the office around 6-7, even if I still have work planned for today. It's not really early but it leaves me a couple of hours with my kids (I live really close to work, like a 10 minute motorcycle ride).

2. If I still have urgent work to complete, I just continue work from home after they are in bed and I've spent some time with my wife. If you have to work late, this makes for more productive midnight-crunching, because you're at home with your family, you've eaten a proper dinner and not some pizza, had a shower, etc.

3. Once a week I leave work early (around 4pm) and pick my kids up from day care, and I dedicate my entire afternoon to having fun with them. Of course not every early stage start-up would agree to that, but luckily both my last employers were start-ups founded by young dads as well, and they were understanding, and do this themselves as well.