You haven't mentioned her complaining of any back pain? Which for me would initially rule out Sciatica, stress fracture, L-spine damage, disc problem. Not to say that isn't possible, but I would be inclined to look elsewhere first and come back to that if still at a loss.
Could possibly be a tight piriformis muscle? It's a small muscle, deep inside the bum/hip joint. It sits across the sciatic nerve and when tight, can often be the cause of sciatic type pain. However you haven't mentioned your daughter complaining of shooting/tingling pains down the legs which is a good sign. It's a lateral rotator, so when it contracts it turns the leg out from parallel, but it's also involved in extension - when the leg is behind. In a backwards walkovers - she bends back, the leg is in extension and the piriformis is contracted, then the leg will kick over stretching the piriformis which, if it's too tight, will be sore. Would definitely cause pain across both cheeks.
This would be a good stretch to try, and will also stretch out the gluteals which are probably tight too. She demos a basic version, and then a progression. I find the progression version to be most effective and stable.
I am studying to be a soft tissue specialist right now so like @
Mamarazzi, I would also recommend some deep tissue sports massage if at all possible. A bit tough because she's such a young athlete, but if you find a willing therapist then there is PLENTY of good work that can be done through a towel, avoiding the therapist being hands on. Ice is good too - alleviates pain, and reduces inflammation. Bag of frozen peas, for 15mins after training, make sure she moves it around the sore area, not holding it on one place.
Please do let me know if the piriformis stretch helps her, or not at all! It is all an education for me :)