An option is a "pedal-forward" bike with bucket seat. It will be absolutely nice to the butt, just not sure it comes in 7 year old size.
https://bicycleman.com/crank-forward/
I have to caution that true noseless saddles would only be comfortable if you have a large setback setup (pedal forward, semi-recumbent, recumbent etc). Otherwise, it can still hurt the butt. At least hurting the butt is less risky than hurting the middle parts.