"Butt-tops"

Yesterday, after swimming lessons, Ella didn't want the lady behind us to see her getting changed because she would see her "Butt-tops." She thinks that is the techincal word for butt, which she is confusing with buttocks. Her teacher sometimes tells the class to sit on their buttocks, but ella cannot be convinced that it is not "butt-tops." I think she has the cutest little but-tops on the face of the planet!