I hope my answer to this topic is not rude. But somehow there is something wrong if already 6 and 7 yo are afraid of getting back pain or other health problems when they are just starting the long journey of gymnastics. This is to me a big coaching no no.
Somehow also your coaches should know and also teach how a BWO and other skills should be done without harming the gymnast... It is super easy to do a BWO through the back. But normally coaches should have an eye on this and go for the correct technique. For this there should no PT be needed.
From what I saw from other coaches those girls don't figure it out in time or without harming their body. Please make sure that your coaches know what to do. So many injuries can be prevented by proper technique and teaching how to listen to your body. There are also a lot of ways to teach being tight. Better go back some steps and work on the basics, then ending the journey to early.