Instead of buying a $200 mushroom, I would build one or have someone build you one.
There are plans on the internet.
One of the problems about buying a mushroom, is if your gymnast gets past level 5 he won't really be using it much unless he uses it at home and more than likely by that time, he will be in the gym so much (4-6days/week@4hours/day) that he won't have time or inclination to at home.
For beginning gymnasts, doing a lot of work in a bucket is important besides the ability to hold the front, rear and side planks with proper wrist lean.