I've done it. I've changed something. Whatever else happens from this day onwards my parliamentary career will have some meaning. My obituary - short, dull and largely unread - will have at least one significant paragraph.

Politicians tend to have at least one thing in common. Whatever the real reason for going into politics - being bullied at school, inability to hold down a real job, incurable egotism - all of us give the same answer when asked (usually by schoolchildren) why we wanted to be an MP. We want to change things and make them better.

In terms of actually changing the law of the land, this is incredibly difficult. We sit for hours listening to ministerial statements and speeches about future legislation. We are corralled into committees to pour over the detail of bills that may be of no interest to us whatsoever. And if we ever come up with an idea of our own and table an amendment, you can be sure it: isn't party policy; won't get called by the Speaker; is brushed aside by the minister; or a combination of all three.

That's one of the reasons why constituency work is so satisfying. Even if the problem at the surgery is drains or hernias (and sometimes it's both) you feel that by acting, by writing letters, by harrying officialdom you can actually get something done.

So what is Cameron's law? It may not be the Belisha Beacon or the Anderson Shelter (both named after government ministers in the first half of the last century) but it means a lot to me.

The government is pressing ahead with its plans for child trust funds or "baby bonds" as they are also known. Every newborn nipper will have special bank account opened at birth by Gordon Brown, together with a cheque for £250, courtesy of the taxpayer. Parents, godparents, friends and guardians will be able to contribute a total of up to 1,200 pounds a year to the bond and all the interest and capital growth will be tax free.

Aged 18 the budding little capitalist will be able to crack open the account and, according to your view, invest in worthwhile training, buy a car, put down a deposit on a flat - or blow the lot on lager, night clubs and 18-30 style benders in Falaraki.

(The hardcore cynic would point out here that the bond might, just, stretch to paying your top-up fees at university.)

Now for my bold initiative.

Sitting on the committee that examines the bill in detail, I tabled amendments to abolish the limit on contributions for baby bonds held in respect of disabled children. Under my plan, parents, friends, the local community, whoever, could pay as much into these children's trust funds as they liked.

More importantly, under my plans, the parents of these children could dip into the baby bond before the child reached 18 to pay for some of the many things that disabled children need. My list included: night nursing and respite care; the hire or purchase of equipment; and the payment for specialist medical or palliative care.

I declared my interest (my son, Ivan - aged 22 months - is severely disabled with cerebral palsy and epilepsy) and ploughed into a speech about this wizard scheme.

It works on many levels.

More disabled children are surviving birth and living longer. If they are to lead more independent and fulfilling lives they need the resources to do so.

Families with disabled children face massive extra costs in terms of childcare, specialist equipment and support. Bath hoists, stair lifts, wheelchairs, special cars - the list is endless.

While the NHS and social services help with some of these things, many are means tested and the waiting lists can be horrendous.

It's a well worn joke among these parents that the new standing frame, wheelchair or buggy turns up just as little Johnny has grown out of it.

Parents, friends, and others all want to help but don't know how. Contributing to one of these, hopefully bulging, trust funds would be just the thing.

The more I think about it the more I believe that these funds could make a real difference to the experience of families with disabled children. And for those families that cannot access help and support from others, the government would always have the option of topping up their funds directly.

All of this goes with the grain of the new way of supporting families called "direct payment", where the social services department give the family the money and let them decide what basket of services to purchase.

As ever, I think what swayed the minister was something that I didn't say at all, but that in a way followed on from what we had debated. Imagine the disabled child in poor health, desperately needing some complex operation only available in Houston or Boston and that costs thousands of pounds. What use is a baby bond that doesn't mature until you reach 18 if you are going to die at 16? Under the rules these things can't be traded or sold.

So the minister - Ruth Kelly, the financial secretary to the Treasury and new love of my life - uttered the words I thought that I would never hear. In fact I was so shocked that I can't remember what she actually said. It must have been something like "good idea ... going to take it up ... will bring in regulations to implement the honourable gentleman's scheme ... hope he will formally withdraw amendments so that officials can draft new rules, etc etc."

Anyway, the disabled child's trust fund is now in formal gestation. The birth could be announced as soon as the report stage of the bill and, as father, I will be there cooing and swelling with pride.

· David Cameron is the Conservative MP for Witney