mine have quads but we always follow the mfg riding age restrictions. little kids and big quads are a bad mix.
cycles are good but for me if the kild isn't old or skilled enough to ride a bicycle without training wheels then they shouldn't be on a motorcycle. i know lots of people put training wheels on a cycle but as far as im concerned it really teaches them the wrong way to handle one.
it pretty much boils down to your comfort level on what they ride.