If when she is laying down she offers her bellie for rubs, then maybe when you know she is going to offer the bellie, have a treat already in your hand and as soon as she slightly rolls while lifting her thigh, put the treat near her nose to follow it. This way, your using her natural already in action movement that's needed to roll over If she does roll, mark it with a clicker or what ever you use to capture a behavior and wait for another opportunity.
I tried teaching my boy roll over but he did as your girls does. I never did teach him. So I don't know if that will work. Wished I had thought of trying this back then. I did teach him to bow using a behavior he often offer.
Good luck, it sounds fun.
"If you can't see his soul when you look in his eyes, then you need a seeing-eye dog"