A metric major fuck ton.
If you truly want to be present and have an active role in your child's life, growth, development et al it's going to force your priorities to shuffle around.
If your "startup" is still a priority (but not above your child) you aren't going to be in the office from sun up to sun down. Period.
You'll have to make up those hours later (when the child is sleeping) however, in months 0-24, that's really tough as most children (at least not mine) didn't sleep well during that phase.
During those years, and for me it spanned 5 calendar years given multiple children, working late at night was a luxury, not an option I could plan on.
I however, disclosed that when taking a CTO position at a company that just wrapped up a ~$20 dollar round. It was transparent to them and set the right expectations.
I do acknowledge that a company in that stage albeit not out of the woods, is not in the same position in the seed, Series-A phase.
If you look in my comment history I gave a "day in the life of" for my schedule on how I manage running a startup and keeping my family, health, self priority #1, #2, and #3. It's buried in there somewhere.