Hello and thank you for using HCM. I carefully read your question and I totally understand your concern. Don't Worry. I think it is nothing serious.
Starting with her very young age I exclude cardiac chest pains. The only cardiac problems that we might expect in this age would be valvular congenital disease, but they have other kind of symptoms, typically not stabbing chest pains. A chest pain might generate from different problems. As you have assumed it might be a musculus-scletetic pain. The fact that it generates when she is training might be related whit it.
But chest pain might generate even from abdominal organs like stomach,
cholecystitis,
pancreas ect. Each of them has some characteristics of pain but they might be irradiated to the chest, in diaphragmatic position as you explain.
But it might be a simple "nervous pain". They generates from nervous system, like stabbing, fulminate pains. They just come and go without any damage.
So if I was your treating doctor, if this episodes persist, just make a simple chek up with a physio-therapist for muscular problems and an abdominal echo. If every thing is all right, it will soon pas and she will not feel this pains any more.
Hope I was helpful. Best regards.