I have my oldest daughter set up on a 96 Jag 440 liquid, she went through the safety course last year and was all geeked up when she got to ride her own sled with dad and mom on the trails. My youngest daughter is on a 06 120 Polaris, just installed a power intake kit and smaller gear 34t from recmotor.com she can't wait for snow. Now my 2 1/2 yr old son will be stepping into the 98 kitty Kat. My kids have ridden pretty much their whole lives, in the blood ya know.
Sure is fun watching them grow on the things and stepping into actual trail riding.
One thing I always stress is safety.
Also to get them started use a long rope 50 to a 100 feet and tie it to one ski let them ride in circles until they understand throttle and brake controls. Have fun.