I do agree with that last post Amber - the amount of children are to an extent immaterial.
The problem is that as far as I can see money has been flung at this problem for years now and it has made very little difference: if anything, the problem has excaberated. I am not actually a believer in the old "they had a baby to get a house" stereotype, or that most families on benefits set out to spend their life on benefits.
I think a "typical" case is a teenage girl who does moderately well at school, probably with quite a lot of pushing from teachers (magic 5 A*-C targets) and goes on to do some form of further education. In many cases, the spoon-feeding environment of school has meant the students are ill-equipped to deal with the more independent attitude demanded from them from college and after meeting a boy at college, becomes pregnant, often at the age of 17/18/19. The pregnancy isn't planned but it is celebrated. A second child often follows the first fairly promptly and it seems that it's that second baby that pushes most couples to their limits and the relationship falls apart, leaving the girl with two tiny ones. Obviously, she didn't plan any of this and nor does she plan to be on benefits forever and makes plans to go to college or university after her youngest is at school. Only by then she's met someone else, or she realises that the job she had such hopes for actually doesn't pay that well.
I have a lot of friends who that exact story applies to, give or take a few changes. They are all "nice" girls. None of them meant to end up on benefits long term, but they all have. They all, from time to time, get fired up with something and start at college or university but then lose interest. And the truth is, they have nice lives. There isn't a LOT of money, sure, but there's certainly enough and they live close to each other, and to their mums and sisters and cousins. The fathers stay in touch: child maintenance is touch and go as the father's work is touch and go but they'll give the mums £50 when they can afford it and take the kids to McDonalds when they can't.
I have wished many times I'd made different choices, and I really do mean that. If I could live my life again, I know I'd do it "their" way.