Hi Suffolklatch
I'm presuming by the fact that you're doing chest physio that your child has some kind of chronic respiratory condition?
Around the age of 2 or 3 is probably the most difficult time of life for getting children to do airway clearance and the key is to try and make it as fun as possible. For that reason I think bubble PEP is an excellent choice, it is important to make sure you are encouraging some kind of fast expiration between sets of bubble blowing i.e. coughs, "huffs".
In my experience I have tended to use acapella (or other PEP devices such as flutter) in slightly older children, from around the age of 8, although sometimes earlier depending on individuals. My concern at 2 would be how compliant a child would be and, on a technical note, whether they could generate good enough expiratory flow for the acapella to function correctly.
Whichever method of airway clearance you choose, I can't overstate the value of cardiovascular exercise as a part of any airway clearance routine. Not only is it very effective but generally getting a 2 year old to run around a garden is far easier than getting them to blow into a plastic tube!
Do remember that your physiotherapist knows the details of your child's condition in far more detail than anyone here, so their advice is always likely to be the best.