In reply to Bingers:
The weight of the bike is probably one of the most important factors to consider, followed by the quality of components. Forget rear suspension, for the average kid all it does is add weight and complication, without any gains in performance. If you are only riding tow paths and lanes, then even front suspension is not essential, but a reasonable set of suspension forks helps a lot once you go onto rougher terrain.
My son had an Islabike Creig - cost nearly £600 three years ago, but sold for £300 before Christmas, so only really cost us £300. A £200 bike would probably not have lasted three years!
When looking at a bike, try picking it up. If you struggle, its too heavy! Are the components made of plastic or good quality metal from recognised brands?
(I'm a MTB coach and Bikeability Instructor and spend loads of time with kids who struggle with cheap, poor quality,over weight bikes)