I don't think if people really don't like children they could tolerate 5 just for the benefits, I think it's more likely people start having children or rather don't make much effort not to have them, get into that lifestyle and carry on.
People say 26k isn't much, post-tax it is, and more to the point, if you are someone with fewer qualifications, or large gaps on your CV where you had several children, or haven't retraining so have out of date knowledge, there is no way on this earth you are going to be able to earn that type of money, plus pay for childcare (even just 20% of it) for several children. So you are locked into the children/benefit cycle not as a great alternative for a fabby high-paid job with international travel, but as a realistic alternative to working in a high street shop for the minimum wage.
I certainly have several friends who do their calculations carefully when deciding how much to work- which varies from none (although that friend did go back to work when youngest was 6 and I did encourage her to do so) to minimum to claim tax credits- at the time it was the 16 hour rule, but I think that has changed.
However, I would still hate to live like that because 5 is too many for me, and even though I only get about 26k take home (so after tax) myself at this time point, it's likely to increase.
I think benefits are likely to get cut and the rules harsher and given you can't undo 5 children (and most likely they are loved and wanted too) it is a precarious place to be.