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.
Kudos for making extra time for your kids. I plan on doing the same.
Tho mine wasn't a "startup", I started a web design and hosting business when my son was 2, I worked ALL the time for a good year or 2. I had the luxury of working from home so I didn't really miss big moments, but I did put a serious strain on the relationships between myself and my family. I was even glued to hosting issues when we were on vacations, sitting at my laptop a couple hours a day.
If I could go back, I would definitely give myself 8-10 hours a day max, be home and present with my family by 6. On days where I needed more time, I should have woken up at 5.
I added probably 6 hours a day, 5 days a week, to be more productive back then, but knowing I had those hours, I probably slowed down 2 extra hours a day, and those extra six hours were tired hours, so I was probably only productive for about 3 of them.
The way I look at it, I probably maybe gained an extra week of work time a month back then, and every project I was working on I could have EASILY asked for 1 more week to finish.
Anyway, regret sucks, and looking back, I should have worked smarter and not harder.