How about adding some kind of birth control into a yearly or even monthly injection? People can have all the fun they want without fear of some 'accident' that ends up with fines, jail time, or whatever punishments for having too many kids. If a couple wants a kid for real, they can spend some time doing foster care to see if they really want to go through with it and if they're still set on it they can go through whatever legal process for papers letting them skip their birth control injections. If the couple can't have their own child for some reason, they can adopt, go to a sperm bank, or stay as foster parents.
How the birth control is enforced would be anyone's guess.
How the birth control is enforced would be anyone's guess.