RCKen is telling you right. Most engines idle at about the same speed regardless of size. In fact, a large diameter/low pitch prop will actually act as an airbrake and will allow the plane to slow down nicely.

Get the .25 and use a 10-4 on it. You'll have one sweet-flying trainer for your boy.

When he gets ready for a sportier plane, take half of the dihedral out of the wing, add ailerons (if you didn't in the trainer mode), make it a tail dragger, and change out to a .32 engine. It'll still be docile on landing and slow flight but will be moderately aerobatic and fast.

Dr.1