20 year old Yamaha Sno Sport if you can find one! As fast as a Bravo but more rider forward like new sleds and far better suspension - put on a pair of plastic skis and your good to go.
Bravos / Enticers (250 / 300 / 340) are generally bulletproof and cheap but not much suspension in either. BRP Tundra (new style or older Tundra II) is also a good choice and generally trouble free - newer style is more like the REV platform.
Just like Couch said, Look for a Yamaha SnoSport or SnoScoot, a little hard to find but perfect for the Jr sled riders, try to find one with a bad track & do a 121" conversion. Good luck it's great to hear your getting them into the sport!!!!