My guess is that your daughter is shorter than you are so that the harp is balancing differntly on her shoulder than on yours. Try putting something under the harp to prop it up a bit, and see if that makes it more comfortable. If your daughter is shorter, I think the stool may also be too short.
I’m relatively short but most of my height is in my trunk (short legs). I need my harp to be higher in order to sit comfortably. My Thormahlen Serenade has taller legs, and I play my teacher’s Prelude, a lot taller, when I take my lesson. Both feel weightless. The only harp I struggle with is my teacher’s concert grand pedal harp, which just feels too big for me to balance properly.