I guess it's different in the UK, but in Germany since many kids cycle to school, they get an annual inspection by local police, and they check lights, brakes, etc. If the inspection is passed, they will get an official sticker on their bikes, which makes them super proud.
The side effect is that kids bikes are very well equipped, with full mudguards, dynamo lights, chain cover, etc and the used market is quite cheap. My kids have a Puky Crusader 20 and a Winora Dash 24, for 75€ and 50€ respectively...! Their lights are better than mine!!
An MTB like dad's is sure nice, but mostly for the parents. For the lame off road we do, they prefer their multi purpose bikes that are easier to cycle on the road...