I agree that something needs to be done, but it can't be forced, just encouraged, so I've chosen YANBU.
I agree with another pp that 2 successful pregnancies or a maximum of 2 children, whichever comes first would be more realistic/ easier to do.
Again, I would exclude multiple births from that, ie, one child, then triplets would still be counted as 2 successful pregnancies, and wouldn't be penalised.
I would use tax breaks and other incentives to achieve this, as banning it outright won't work, and I would penalise those who ignore the policy, eg, 3 dc from 3 separate pregnancies with penalties such as higher taxes, etc.
I also agree that education is key, and that men who go around fathering dc with different women, ie, more than the suggested limit of 2, should also be penalised though tax and by having to fund each child properly.