You can get that from the rich as the other person said but that kind of redistribution should be considered on it's own regardless of this matter.

What i'm advocating for is simply internal child related distribution. You don't take it from the states coffers. You for example simply carefully raise the taxes of the childless and lower the taxes of those with kids (probably not directly proportionally to the number of kids) in the process retaining your original government revenue.

You don't have to go overboard with it because realistically any effect drags drastically but you do slowly keep upping the difference in benefits untill you reach either replacement rate or something slightly below replacement in a way that doesn't constitute an outright crisis.