If they have the freedom to choose otherwise, they will. The global fertility "crisis" is simply individuals exercising their choices.
Kids are really expensive, and if you want people to willingly have them outside of accidents, you're going to need to pay them a lot of money.
There's no amount of money (that society could ever afford to pay) that could convince my wife to have children.
Rising kids also has a time, effort and opportunity costs which are not easily offset with money. I don’t think there’s a way to frame modern parenting in a way where it „pays off” in the same sense as it did in the past. As of now, it’s essentially a hobby.