I don't know that I would bother listening to the "recommendations" too much. What I WOULD do is place the child on the machines you are considering. If there is about 3-4" of space underneith them when they stand on the pegs, if they can reach the brake(s) and throttle controls properly, if they can turn the handlebars without being in an awkward position, then you've found a machine they fit.

This doesn't necessarily mean they can operate it, however. I have three son's and one 90cc machine. My oldest son is 12, 5'4" and 110 lbs. He drives like a pro and "outgrew" the capabilities of the sp90 within a few months. He now drives my 400 cat while we wait on getting him a machine of his own (probably in the 250cc class). My 8 year old can handle the 90cc machine just fine, and is slowly learning the skills he needs to do some serious trail riding. Just a year and a half younger is my 7 year old. Physically he's large enough for the 90. He can reach all the controls and is tall enough to "move around" on the machine. Because these small automatic tranny machines need to "wind up" a bit before they move, he has trouble operating it and has flipped it while turning. He's not allowed to ride it now, until he gets bigger.

The mistake I made was not spending as much time choosing a kid machine as I did choosing my own. Take a look at what's out there, and ALL the features on it. Many parents opt for the badger or little Honda because they have MANUAL transmissions. This pretty much eliminates that "winding up" problem and allows them to remove the shift lever with the machine stuck in first gear until their kids become more capable. If you look around these forums you'll find ton's of advice on machines to buy, but my guess is you are right to be looking in the 50cc range for your children's size.