If you are considering prostheses it sounds like you might need to think in terms of hip disarticulation prostheses, especially if they have no hip extension. However they do need to have good trunk control so a lot of work would be needed in this area. Pelvic tilting is required to activate the prosthesis. In a single leg hip disartic. this is extremely tiring and from my limited experience they have usually mobilised without a prosthesis with crutches as it is more energy efficient and much faster. A bilat. makes it much harder still. Havve you considered wheelchair for mobility?