Get the lightest bike you can find within your budget and don't be afraid to look at used bikes. High quality and low weight usually go together. Lots of adult buyers forget that younger kids don't have the muscle mass they will later gain in adolescence and don't take into account the weight the poor kid has to propel. As most adults know, it is a lot more fun to ride a lighter bike than to ride a real slug. You can always buy a second set of road tires as spares for the knobbies that come on most MTBs. It will make riding on the roads much easier.