First of all you’ll be really happy with all those babies when you turn 80 and society hasn’t fallen because the average age is 70.
Second, when childcare is free more people can join the workforce, this increases taxes and productivity.
But maybe the most important it will make society more equal. When you’re poor working almost doesn’t make sense if you have to spend 80% of your salary on childcare. So more people don’t work, these people are usually women. That really doesn’t help with an equal society, because a big group can’t really work so they can’t really climb any ladders.