Miss Puddling Bowl,
Your advice is brilliant based on just 3 CF's. Exercise exercise exercise. Children hate sitting still and the evidence is pointing us towards exercise. Who cares what really, so long as the child huffs puffs and coughs! PS2/3's are great and one way our CF's can enjoy sport without the embarrassment of coughing in front of their teammates which seems to be an issue with the girls I see aged 8 upwards.
If the non-compliance is surrounded around the PEP or if physio means PEP to this child (scary) then change her perception to physio is clearing the chest, however she wants. Form a plan, contact local sports places or investigate charities that may fund a PS3 for the hospital. They'll be lining up for physio.