I think people are missing something important here. There's a great deal of difference between what people should do and what they end up doing all the time. Assuming that people should have fewer children, especially when they can't afford to keep them really, then we should encourage them to be careful and to plan more sensibly. Thats fine and proper. But what happens when they aren't careful, and do have another child? Do we punish the parents for their "mistake", or do we support them now that it's too late to do anything about it (short of a forced abortion)?
People seem to be adopting an attitude that the parents and family should be punished. That's a similar attitude to, say, the one we saw around teenage pregnancies up until the 1960s. Any teenage girl becoming pregnant was stigmatised (if people found out) and so to avoid the shame of being discovered people would run away to back-street abortion clinics. This was often dangerous for the expectant mother. Alternatively if she chose to keep the baby she'd be an outcast, or certainly looked down on by others. That had the potential to end up wrecking not only her life but that of her child's. Yes, she should have been more careful, or more responsible. But she wasn't -- and it's too late to condemn her for that. Instead, we should -- and have -- be more understanding, and provide her with the support she needs to care for the child, or to have a safe abortion.
I think a similar idea should apply here, too, in the case of Child Benefit. Yes, we can encourage people to have fewer children. But once they have, cutting off the support they need damages that entire family. Their mistake, you might say. Yes, it was the parents' mistake. But it wasn't the child's. And the child doesn't deserves punishment, and should have care and support.
This is why I am opposed to a cap by number of children. Not because I disagree with the principle that people should be more careful -- but because once they've had the child it's too late to tell them so. Except for next time, perhaps. But in the meantime we have another child that needs support and cannot be ignored. You can say "tough" to the parents all you like. Try saying the same to the baby who had no say in the matter.