J.M.
I know this is going to sound a little weird, but I had this problem with my daughter. It wasn't that her stool was hard it was the way she was sitting. When she sat she would sink her bottom toward the water and that pulled her apart like she was ripping apart. I taught her to prop herself up using her arms or lean forward just enough to to keep from falling in and that helped. Also, if you rub the small of their back with downward strokes it helps massage everthing out smoothly. (Works for adults as well)