Maggie,
It would actually equate to a lower salary, because automatic Child Benefit (£33pw) would be factored in. Until recently, there would also have been an amount for Child Tax Credit.
I agree with you that it seems quite a lot, especially if the children are young. However, when the children are older, they cost a fortune and the figure just highlights that. These days parents end up being financially responsible for their children than in the past.
I don't know how the calculations have been worked out. Children have to stay in education until they're 18, but the Educational Maintenance Allowance for Sixth Formers has been scrapped. Sixth Formers don't automatically get school transport paid and they often have to pay for their own books. Since student grants have been abolished, it's almost impossible to pay for university without parental support. If the child doesn't go to university, but can't find a job, he/she gets so little in benefit that he/she will almost certainly have to continue living at home and be supported by parents. Maybe all this has been taken into account - I'll check it out.
For younger children, maybe it's assumed that both parents will be working and the children will need childcare. Average fees are about £600pm for day nurseries round here (and can be over £1000 in London). Again, I'll check out how it's been worked out.
In any case, the government doesn't use those figures. They use 60% of the median income, which is about £11,500 and it's that figure they use when they say that 4 million people are living in poverty. Four million households have incomes below that figure, but the government doesn't differentiate, so a single person who's paid off a mortgage could probably cope, but I think a household with children and rent/mortgage would really struggle.