In addition to the forward flexion you need to be very careful with spinal twisting. In general those with ostoepenia are at less risk then those with osteoporosis. In addition you need to encourage more weight bearing exercises, as Pilates does not provide enough stress on the bones to deter more bone loss.
You have many suggestions to help you.
I believe that the exercises will help slow the progression of bone loss; just, start out slow and easy in the beginner format with less ROM. Keep a close eye on the clients and make sure to keep open communication with them for any signs of difficulty and poor form. Do not let them lift both arms or both legs simultaneously, either. Have you tried using thick, soft or multiple mats on the floor?
Not to mention, how many of us have disorders that we are not aware of?
Good luck and enjoy keeping the seniors healthy.
I third Peggy. Because of decompression to the spine, and often poor postural habits/weak muscles, working in neutral spine (spine twist, half rollback, toe taps, bridge, single leg stretch & others without curling up, quadruped and side lying positions) and in extension will be the safest and most rewarding. If you are worried about wrists (in quadruped) you can modify the exercises by having them on their forearms (and triple the mat to protect the knees).