If you can continue riding it for years it's a bicycle. No room for snobbery on my bike. You might try on the (really very friendly) forum yacf.co.uk for advice, too. There's a subsection called Kidstuff where people may be more clued-up about your options.

Here's a translation of the previous page: http://www.bakfiets-en-meer.nl/2008/10/26/workcycles-and-bakfietsnl-win-in-kassa-bakfiets-comparison-test/

Upgrading all the brakes is most likely to be 30 quid for parts, and a job you can do yourself without exhausting your vocabulary of swearwords; or might be an engineering impossibility, depending on what is actually stopping the wheels now. My guess is that those are modern hub-brakes on the front wheel and that you wouldn't need to do anything anyway.

I've seen bakfietsen around Oxford though the only people I know who use one for their 4-year-old are in Cambridge. Even here in London there are some trailerbikes about, in these recessionary days.

(I like owning a tandem but it cost a lot to get from ebay condition back to rideability, and it gets much less use than I expected. Nonstandard bikes tend to do one thing well but take up a lot of space when they're not doing it. So, for myself, I'd be going for a baby seat now and a detachable trailer later.)