Who diagnosed the hypo-mobility in your child ?
I would have thought a referral to a physiotherapist would be indicated.
Taking your child to a soft play area or park, walking the dog, going swimming, playing in the garden and going on bike rides are great ways that you can promote muscle strength, balance, proprioception and joint stability.

Have you sought the advice of a medical professional at all? Do you know what has caused the problem. Maybe you could start by massaging her limbs. If had been a muscle problem I could make some suggestions. Are you able to very gently bend her limbs at all, even just the slightest little bit. However seek medical advice first. You may do more harm than help.
Maybe seek the advice of an organisation which deals with this type of disability.