I am also pro child limit laws. I would suggest nothing too strict, just impose a tax on the third child and up. We know from psychology that little restrictions and the mere fact of a 'cost' would reduce the intrinsic desire of people to even take an action, in this case child bearing. Furthermore, I think a tax is a not an extreme measure, considering the fact that you would still be allowed to have children if you want. But do keep in mind that the poorest, less educated are often the ones to have plenty of children, which ends up harming the family, this might be good for them.