Seriously. Kids awake 12 hours/day when they're young, overlapping with work during the week which makes that calculation easy. 12x7 = 84. You can kind of do other productive stuff during that time, but not much. 8hrs/night sleep is 56 hours/week, assuming you don't need some time to wind down for sleep (most do).

84+56 = 140. 168 hours in a week. That's 28 hours left for everything that's not kids, work, and sleep. Fixing stuff around the house, keeping your relationship with your partner healthy-ish, prepping for the next day so you aren't constantly running late to everything, cleaning, entertainment, learning, everything. In practice you'll have an average of 2hrs/day that's actually free, maybe, without neglecting sleep, and they'll be your worst, lowest-energy, lowest-motivation hours (work gets all of the best hours 5 days/week, kids do the other 2 days)

Maybe don't have children unless you can afford it? I've put off having children until I have the financial resources to raise them. What is stopping others from doing the same?

It's healthier to have children at younger age.

Irrelevant point. Whether it's fiscally prudent or not is what's at stake. Generally it would seem it's not a good idea (financially) to have kids. If you must, wait until your career is at a point where you can be assured you won't have to raise them in near poverty-like conditions.