Part of the problem with making sense out of polyphony comes from historically different ways that manufacturers defined it. Some keyboards used (do any current digital pianos still do this?) many different voices to create a single sound, so you could have a case where it takes 4 or 8 voices to create a single note in the keyboard thus reducing the 32 polyphony to 4 or 8. I am sure that the casio ap65r does NOT do this.
On most current digital pianos if they advertise 32 note polyphony it either means exactly that, or it may mean that the better sounding stereo voices have only 16 note polyphony. Most of the casio pianos cut the polyphony in half for their stereo grand piano sound. If you look at the manuals available here: http://world.casio.com/emi/download/en/manual/
you will see that the 64 note polyphonic instruments reduce the polyphony to 32 when playing the stereo grand piano sound. I'm not sure if the ap65r does this. Even if the ap65r did have only 16 note polyphony on it's stereo grand piano sound, will a 10 year old notice the difference? I doubt it. Would a 10 year old notice if you switched from the 16 note polyphonic stereo sound to the 32 note polyphonic mono sound? Again, I doubt it. In fact, my 9 year old insists that he likes the mono sound better on my yamaha digital piano! It drives me crazy that he can't hear how much better the stereo piano sounds. Kids!