It is amazing how much more expensive children become as they get older. As babies, my children didn’t seem very expensive. Cots, prams and highchairs could all be borrowed from friends; nappies were my only real expense as the children both looked so sweet in clothes that their older cousins had passed on to them; and they had such birdlike appetites that what they ate didn’t really affect my food bills.

But now, aged 4 and 7, and having started them at a local primary school, I am noticing that the cost of bringing them up is increasing at an alarming rate. As well as two sets of uniforms, there are also the winter coats, bike helmets, bike lights, violin lessons, ballet lessons, karate lessons, school dinners . . . the list seems to be endless, and this autumn I have felt as if I have not been able to turn around without signing another cheque for about £79 on something that the children really need. I am not lavish and they are not spoilt, but I want them to have the sort of things that I and their father grew up with such as music lessons.

So last month, frustrated by three years of failing to persuade my exhusband Joe, to contribute in some way to his children’s upbringing, and feeling slightly anxious about these escalating costs, I called the Child Support Agency (CSA). They were very supportive, and quickly ascertained how much he was earning from his part-time work. Since we split up three years ago, Joe has never paid for anything for the children, who live with me but spend every other weekend and some of the holidays with him.

Joe’s parents are very committed, loving grandparents, and they have always bought the children their shoes, but otherwise the entire burden of financial responsibility for the children has come down to me. I work as a freelance journalist, and like a lot of mothers am adept at running straight from work into watching my son reading in assembly. Juggle? I can do it in my sleep.

So after three years of accepting this responsibility alone, I felt almost excited at the prospect that we might be about to share it more equally, because the CSA was set up to ensure that nonresident parents should contribute financially to their children’s upbringing.

They called me last week to tell me that, based on his earnings, Joe is obliged to contribute £5 a week to the children. I was making tea for the children when they called, so instead of crying, I just had to laugh.

That pathetic, insulting figure of £5 enshrines all of the failings of this shambolic system. Indeed, a report earlier this year for the Public Accounts Committee (the parliamentary select committee which checks to see that government expenditure is effective and honest) claimed the CSA will have cost more than £850m when it is replaced by the new Child Maintenance and Enforcement Commission (CMEC) in 2008. The report went on to maintain that just 2% of what nonresident parents – and let’s face it, that’s largely fathers – owe to their children has been collected by the CSA, and 60% of money owed to resident parents is now “uncollectable”.

This is a system that has cost the taxpayer a fortune: the Department for Work and Pensions paid £91m for advice on reforms between 2001 and 2005, but only a third of that money can be accounted for. Bizarrely, despite these manifest failings, CSA staff have been paid more than £25m in bonuses over the past five years, even though there is a backlog of 240,000 cases and more than £3.5 billion in uncollected debts on its books.

Two years ago, during his stint as work and pensions secretary, David Blunkett described the CSA as a “shambles”. And Edward Leigh, the chairman of the Public Accounts Committee, earlier this year described the reform of the CSA as being “one of the greatest public administration disasters of recent times”.

It was therefore with a weight of expectation on her shoulders that Janet Paraskeva, the former chief executive of the Law Society and currently the first civil service commissioner, was last week appointed chairwoman of the CMEC. A Jewish lesbian bringing up two children with her lover, Paraskeva is not expected to be sympathetic towards fathers who fail to pay up. But I am not sure that this is anything but rhetoric, and anyway, would I really want to send the father of my children to prison over this? It would be damaging for his children, so of course I wouldn’t.

And a lingering spectre of jail does not, anyway, really help those who this issue is really about, namely the children, who seem to have been forgotten in this increasingly ugly and undignified gender war; and since that Fathers for Justice campaigner climbed on to Buckingham Palace, it has been war.

Sometimes I feel like screaming, “I can’t cope with paying for everything on my own” but I know that this isn’t true, because if I don’t cope, then who will? I don’t have an option but to cope, and sign another cheque for my daughter’s school trip or buy a new winter coat for my son when he grows out of his old one.

I am extremely independent now, but it’s a role that has been forced on to me. I am not going to stop Joe from seeing the children because this is destructive for them, but it just seems so unfair that he can simply shrug his shoulders and walk away from paying for them, simply due to the fact that I am willing and able to work hard enough to support them on my own, and he refuses to contribute.

But what if I could not support them? What if I was living on benefits, or a very low income, as is the case for millions of single mothers? Would my children end up on the social care register, due to the failings of the CSA, which allows men such as Joe simply to negate all responsibility?

“When Ellie was small, I was completely broke. We lived on my credit card and I couldn’t have bought her Christmas presents or paid for new clothes without it,” says Sarah Kay, a marketing manager and single mother to her 10-year-old daughter. “Her dad claimed not to have any money. The CSA could not make him contribute anything towards her upbringing, and quite apart from the financial burden, it had a very bad effect on me emotionally as the resident parent. I felt as if he had disowned Ellie.

“I was devastated when they told me that they couldn’t make him contribute to her because I had had faith in the system, but it completely failed my daughter. It was as if he had got away with it, and there was nothing that anyone could do about it.”

I don’t really feel angry with the CSA but there is no doubting the fact that it has spectacularly failed to make men such as Joe contribute to their children. Joe comes from a stable, hard-working family, and is bright and able. There is, however, absolutely nothing that I can do to make him contribute more to our children’s upbringing, and he knows it.

The CSA gave me the option of appealing against its decision, but they also warned me that it would be hard to prove exactly what Joe is earning. I don’t want to pursue a lengthy, acrimonious case that, based on past performance of the CSA, is almost certainly destined to end in failure. I have dropped all claims of financial assistance, and he can keep his fiver; I won’t ask him for money for the children again. I’ll just work harder than before. “He can’t be allowed to get away with it,” my friends say, but the fact is that he can, and he does.

Britain’s top 2,000 schools online

Go to parentpower for the full 2007 Sunday Times survey of Britain’s best state and independent schools. Find all the top secondary, primary and preparatory schools in your area simply by typing your postcode into our online database. You can also search by school name, town, region and education authority. As well as displaying all the recent exam results for each school there are links on Parent Power to take you straight to the latest inspection report and the school website

Free Hamlet download: An outstanding 240-minute recording of Hamlet, with Sir John Gielgud

Explore Education

• Young Times

• Schools

• A Level & GCSE Results

• Good University Guide

• Sunday Times University Guide

• Student

Times recommends

• Upper second loses kudos
• School scores will drop 2% at Key Stages
• Exam results delayed for 1.2 million children