Earlier quoted context omitted.
Your estimate on time to take care of children is ridiculously low.
I was thinking the same. I don't have children, either, but observing my friends and siblings who do would make me estimate this at 40+ hours/week. (Estimate applies to children below age 6-8. Of course, it's progressively less the older and more independent the child grows.)
The bigger issue is that this time is spread out pretty evenly throughout the day, so in practical terms it takes up every single hour that the kids are awake. What changes is that, as they get older, you have an increased ability to do other things in the in-between moments.