In reply to Nic:
If you're prepared to stump up to get on the Islabike bike bandwagon with a decent secondhand cnoc, you can progress up the range selling each one for only a little less than you bought it for 2 years previous. I've 2 kids and on our 4th Islabike and by the time we've finished with Islabikes in a few years, will probably not have spent more than a few hundred pounds in total (to enjoy about £2,000 of bikes at rrp).
Having looked at Frog as well, I preferred Islabike as they have a little more relaxed geometry. Frog seemed to me to be a bit more stretched. Both brands are good though and a key plus is their relative light weight. Cheaper kids bikes are so heavy and not conducive to learning good bike handling.