Making hard rules like these seems to me like the only sensible solution. In fact, I'd go so far as to say that expecting to run a successful early-stage business and being a good parent to two young childen is impossible unless you lower your requirements on one or both fronts. If you don't lower the requirements voluntarily, nature will do it for you. (There are rare examples of people who have extraordinary endurance, but they are by definition extraordinary).

Burnout is not fun and it can put your out of business for months, or even years if you have a bad case. (In my case, it was a year and a half of zero productive work). One of my relatives, after a particularly rough Ph.D, claimed that she will never be able to perform at the same level again. It's okay to be ambitious, but I think too many young and ambitious people let their ambition get the best of them. Learning your limits by crashing into them at full speed is not advisable.

I don't mean to sound negative or dismissive, but I've experienced this myself and seen too many cases first-hand not to care. The Silicon Valley startup culture is in many places outright destructive when it comes to this aspect of life.

More constructively: Setting strict deadlines and schedules that you will maintain unless the sky is falling (change the definition to "no matter what" if the sky is "constantly" falling), balancing your time so you have enough sleep and enough time to yourself is a safe way to ensure that you are working sustainably. This will lower your productivity, though. At least make your sacrifices with both eyes open.