I've said for years that if a couple has more than 2 children, all benefits (chlid benefit, tax credits etc...) should be cancelled immediately (Obviously there would be allowances for people who have triplets etc... or who have one child already and then have twins.. as that isn't their choice). People who choose to have more than two children are fully aware who expensive they are to bring up so if they choose to have more children then they should be aware that they're going to have to bear the cost of them all by theirselves. I know plenty of people who have three or more children happy that they can claim no end of benefits for them.... If these were cancelled i'm sure they'd be less inclined to keep having children.
Didn't they have a one-baby policy in China (maybe still do have) where there is some punishment/fine for having more than one in an attempt to control the population?