Bother I just lost my reply. I really disagree about putting them on a significantly too small bike, seat down so they can put their feet flat on the ground and use it like a float bike yes, undersized no. A bike that is too small is harder balance, pedal and control.
I would also add lots of practice "float style" on the surfaces they will ride on. Let them learn to balance and steer on your local paths until thy are really comfortable before adding in pedals as well. DD "rode" everywhere we went locally for months and this provided lots of natural, incidental practice and lots of confidence on those paths once she could ride.
