A few things to consider are weight and shell size.
The Bell GP2 Youth helmet comes in 4 sizes and will also come in colors other than silver for 2014. The Bell is also very light weight which is important if the driver ever gets in a big crash.
Just as important as the weight of the helmet is the shell size. In my opinion you never want the shell size of the helmet to be wider than the neck collar. The neck collar plays a major role on protecting the collar bone in a roll over accident. If the shell of the helmet is wider than the neck collar the edge of the helmet will make contact with the collar bone.