I bought my daughter an almost new Trek MT 220 prior to the addition of the front suspension fork (2002/2003 model ?). It served her very well (2 years) until she was big enough to handle a 15" frame 26" bike, and is now waiting for use by her little sister.

Buy what fits them and what they can handle without putting them at risk. The $100 you are out after buying and reselling is small potatoes compared to the pain of picking up the pieces after a bad crash.