Well, you are correct in thinking that the behavior is pleasurable to her and in that fashion really is no different than having a treat. While it can be hard for parents to understand or accept, young children do derive physical sensations and pleasure from masturbation. They usually discover this quite by accident, but when they do the behavior is very reinforcing. The most prudent approach is to set a limit not on whether it occurs, but under what circumstances. Tell her that it is permissible only in her room and only when she is alone.
thank you for ur concern !