your suggestion of "socialize childcare big-time" is tried in EU, countries like sweden offers the best support of families and yet are parley getting the numbers up without immigration, the root of the drop is the change of women role in society, finding a political sweet spot is required, if women get a salary from the government when they have more than 3 kids then it might encourage families to take the big step and the women to spend more time doing the real full time job of home care, also families needs help with bigger housing, logistics and home duties when they have more kids, normally that help came from the grandparents but it's not happening anymore because the average age of the grandparents is higher, and baby boomers and "older generations" are assholes by blocking the housing options for their kids, i can also suggest that a adults with no kids can get some money by hour from the government for helping families with many kids as it is also needed sometimes.

solve this: - Housing issues. - Income issues. - Care and time issues.

It's really alarming when governments see their society collapsing and do nothing, if you have such government, remove it, it's a stupid government.