If you want them to experience it as a human body, just add software that makes it feel like they're moving muscles that aren't really there. Also, if you can find the right part of the brain to mess with, make them incapable of noticing something's wrong with their body.

Besides, pneumatic motors are tiny. It's not that hard to add more. Granted, they'd have to carry around a compressed air tank or air compressor if they want to walk around a lot, but that's not going to be a problem if you just want to hug your grand-daughter.