Direct subsidies would encourage the wrong people to have children for the wrong reasons.
A better answer would be to restrict immigration. This would put upward pressure on wages and downward pressure on housing, which together would encourage families and children. There was once a time when people could support a family with one income, and still afford to buy a home.
Low fertility, as well as high abortion rates, and bad mating and parenting choices, can be fixed for free: give back the return on investment for raising prosperous children to the rightful recipients. Give parents all the socialist security related deductions from their children’s future paychecks. That is functionally how it worked throughout most of history, and still does in most of the world.