I've got to go with pwerwagn, but for a different reason. Safety. I've been the parts manager at a large shop in Nashville for almost 20 years, and in my opinion, a mini bike is MUCH safer than an ATV for kids. Think about it, if they crash the bike they fall over, pretty much like a bicycle wreck. On an ATV they hit a bump or a ditch, the ATV starts bouncing, kid falls off and the ATV either runs over them or bounces on top of them. The ATV won't "fall over", it just keeps going. My son is 4 and he's getting a JR50 for Christmas. He'll start of with a set of the new Wheels4Tots training wheels. They're great.
As far as quality of the Eton, Polaris, Bombardier, etc. ATV's go, they're all basically the same. My store sales all three I just mentioned. Most parts interchange. I've not seen many problems with any of them unless they're crashed. That said, I'd still buy one of the Big FOUR (sorry NoCalH. O. PO! Can't put a Polaris in there!). Yes, they're more money, but as mentioned the kid's outgrow them so fast, unless it comes from a BIG family that's been passed down through 3 or 4 kids, used ones are the way to go. Plan ahead, don't be in a hurry. The deals are out there. Be patient.
Just my opinons,
Scott