I treat several children with Arthrygryposis, none of which are the same. I utilise the childs strengths to achieve what I can, no two children will find the same way of moving. However, I did find it useful to look on youtube to get an idea of what these kids were capable of. They are amazing and children will always find a way of moving. If I can be critical of your treatment selections "junior physiotherapist" (without any offence intended) I would comment that your treatment selections are too passive. I know the child needs hip surgery and are by the sounds of it in a boots and bar, but they need to be taught facilitated rolling, watch closely what movements they have available and work with that.
One little lad I have who is 2 (bilateral CDH, bilateral TEV, prev forg leg deformity) now can crawl, with vertially no knee flexion (20 deg) no wrist ext, no elbow flexion etc. AND HE CAN PULL TO STAND AND CRUISE!!! So strengthen, strengthen, strengthen, use what they have and work hard.

And to Warashak, I have not found muscle transfers to work on these kiddies, the theory of muscle transfers and the pathology of Arthrogryposis do not match up. Your joint isn't going to move more just because you have relocated a muscle. We serial cast to gain range (sometimes accompanied with surgery) and then strengthen in the new range with exercises-that is just specific play.

I hope this is of some assistance. I am happy to be more specific if needed