Emily has the acapella. We got one right after her dx at age 7. I agree with the above; the work is done with the exhalation. the resistance it generates (which causes that vibration sensation deep in the chest) is adjustable.
The acapella is supposed to be able to work effectively even if done in various positions (laying, sitting, etc) so as long as your child is able to breathe well for it I am not sure they need to maintain an extremely rigid position for it to 'work' right.
Emily has some trouble with it since the long exhalations needed for it are a challenge for her. (PFT's are a challenge too.) So the acalpella wears her out quickly. However, when she does it right it makes her cough. You can get an adapter piece for the acapella choice to neb while using it. the Choice version can also be taken apart and boiled or otherwise sterilized.