Depends on the age of the kid and the school.
Any exercise is worth getting. We have 3 boys in junior high and early HS. Is brutally hard to get them off of the couch. #1 son has finally become a self starter and is running and lifting weights, but that is the culmination of years of encouragement. The younger twins are required to swim, or run, or lift weights every day. Or soccer or SOMETHING, as long as they do SOMETHING. Mostly all the boys want to do is sit on the couch. An awful lot of the kids in the Jr HS are fat. Is sad. Near as I can figure, a lot of parents suck.
I wrestled (poorly) in HS, and was involved in various martial arts, off and on, in the many decades after. So I have a feel for the various natures of the different martial arts. I've spent time in judo, but not BJJ. I've tried on a couple occasions to get the kids interested in tae kwon do by sending them to camps and classes. Ok, that's the preamble.
Most kids are very whim driven. That is to say what they want/don't want isn't very intense and it's not that hard to get them to change their mind. Imo a kid should be introduced to every sport available. Then see what the kid seems to like and help him/her stay interested in. It's so damn valuable to find an exercise activity that the kid seems to like, that the actual nature of the activity is a distant second in priority. Depending on the kid's age and the BJJ school, the kid might not get much out of it. Or the kid might gain all sorts of skills and self confidence. The important thing tho, is finding an exercise program that the kid likes, and if that's BJJ, open your wallet and make it happen.
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