The acapella can be used by young children but it requires a focused breathing technique in order to be an effective method of airway clearance. My daughter just had a session with the PT at clinic last week working with the acapella (she's had hers for a while but it's been more recent that we want to explore using it more often.) Emily is developmentally/cognitively the same age as your daughter and is capable of using the acapella effectively- the PT watched her do it and worked with her. However, it's not at all 'passive' ...it requires focused concentration to do it right, so it will require cooperation on her part.
Our PT recommends 5 breaths (deep breath, hold to 3, long slow exhale through the mouthpiece) through the acapella followed by 5 huff breaths, then some good coughs, repeat the sequence 3x. The huffing technique is something our kids need to learn eventually anyway to make the best of whatever airway clearance they use, but is vital with something like using the acapella. In Emily's case appropriate huffing technique was more challenging to learn than how to breathe with the acapella and is something we work with regularly.
We were told that yes, it can replace some of her vest sessions if we'd like. Her PT would prefer she still do the full vest session in the morning (and since she does her hypersal then anyway while vesting, it makes more sense.) If we would like, we can try in the evening to do her nebs first followed by acapella on occasion and see how she likes it. I see us doing this during times she needs extra treatments, as well, to give a break from the constant vesting- <b>if</b> it proves effective for her.