OK,
My son rides an 85 now, but I put in my four years worth of racing the PeeWee class and basically, the top four are just about the same. They all cost the same, they all breakdown the same and all require you to buy two if you want to be competative at the National level.
Just because Cobra dominates the Nationals doesn't necessarily mean it's the best bike out there. The results are based on the rider's ability. Cobra invests a lot of money in maintaining that reputation, including persuing these National caliber riders. Why are so many there? Because top riders know the reputation of these bikes. However, I've seen top guys on LEM, KTM, and Polini do just as well. KTM had parts availability problems several years ago, but now I hear that is pretty much solved.
Each has it's unique attributes and I would put your kid on each to see what he/she feel comfortable with.
The KTM is narrow and suits lighter riders better in my opinion.
Polini is a taller bike and has a good clutch system
Lem is really doing great things on their most recent model. When we raced I hated their clutch, however they may have changed it I heard.
Cobra is squattier and has a wide seat that some kids just don't like, however, they build a really great bike and parts are readily available.
Go outside those four racing 50's and you ride at your own risk.
The bottom line is....if you intend to be competative, and I mean race every weekend or twice a week, like many of the mini parents do, you better buy two bikes because this hobby is worse than a "hole in the water that you throw money."
Be prepared to spend A LOT of money on maintenance, or like me, be forced to become a bike mechanic. If, you don't buy two bikes you better buy an equal $$$ amount in spare parts because ALL these bikes break from the wear and tear of a good rider.