It doesn't always work like that. If you want two but you start TTC at say 27 (the age I and many of my peers got married, which is why I pulled that figure out of the air), but on average it takes 12 months to fall, you are pregnant at 28 with first baby at 29.
It's not age 4 they go to school, it's the September before the turn 5. So let's add 5 years on, to be safe. We're now 34. And likely paying for wraparound care and holiday clubs. So you start TTC again because you want 2 kids. Another year, fall at 36, baby at 36 (if it happens because after 30 your egg count drops off signifcantly; and yes I know its possible, had a post 30s baby myself and many people I know have, but its still a factor).
2nd baby not at school til you're 41. And you've spent the last 5 years paying nursery fees AND wraparound/holiday. And are facing several more years of wraparound/holiday. Say til they're both in secondary school, so another 11 years. At 52 you might start getting your life and salary back. And maybe a career if you paused it to be around more for the kids. Not ideal though, when you think about it in those terms.
Put the kids closer together and you could be done with childcare costs at 40-45. If you can afford two lots. Also takes away some of the risk of secondary infertility. And we won't even go into those who start trying in their 20s and still haven't conceived their first in their early 30s and then start ivf etc.
Long way of saying it's more complex than just "time it right for childcare costs". And, why should we have to choose to stretch out that much of our lives paying childcare fees, purely so we can work and pay into the system that keeps failing us? It's broken.