Look what Germany and Sweden are doing for example.
https://www.bloomberg.com/news/articles/2018-05-23/how-germany-is-defusing-a-demographic-time-bomb
After decades of low birth rates, Germany just posted their highest rate since 1973.
More $ assistance to parents (paid time off), low cost child care, free healthcare for kids.
Controlling education costs and giving higher levels of support would also help (ie, aid, not just loans.)
High cost of retirement is also a threat here.
You can't expect families with flat wages to pay ever more for childcare, education, healthcare and retirement and expect them to have lots of kids.
But we're a bunch of idiots who borrow more money to give to rich people, cut support for the masses, then try and keep out immigrants and think that will solve our problems.