Then that's perhaps partly because I could have made my point clearer:
If you introduce some cap on benefits by number of children, then that cap will almost certainly affect those people who have just had a large family but fallen on hard times. You don't want to affect those people, because it wasn't their fault. Good luck writing a system that manages to deal with the tiny, tiny minority of weirdos who might think that it's worth having another child for an extra £60 a week (I don't think that's my idea of a good deal) -- good luck writing that system without also affecting the innocent.
The state benefits system shouldn't judge -- at least, shouldn't judge why people are in the situation they are in. That's what people are for. The state's job is to support those who need help, regardless of how or why they got there, or whose fault it is. We've been getting rid of judgments assigning blame to people in divorce, for example (who cares whose fault it is?) and in general the law has been moving away from moralising. I see this in the same sort of way.
I quite agree with your general point that people should be more careful, should try not to make mistakes and should be more responsible. Once they have made the mistake, however -- too late to judge, now time to support. Tut, chide, try to persuade them to be more careful in future. But, ultimately, once there is a child involved, if you take money away from the parents, you are really taking it away from the children too. And they didn't choose to be born.
Again, please stop demonising people on benefits. This is a problem in so few cases as to frankly not be worth changing benefit laws over.