The only push-back I have on BJJ (as I practice both KM and BJJ) is that there is a huge sport-orientation whereas KM, assuming trained competently, is a combative system such that you'll often get a "practice this slowly and carefully because you can and will cause damage to your partner."

Personally, I have a little harder of a time training BJJ for the reason that everyone thinks it is the best for real-world defense but the reality is many moves open you up to serious maiming in a combative environment. Take for example the typical "get past the guard" move, I most always have the option in a true fight of grabbing your knees and splaying them outward leaving you with an unfortunate convalescent leave, perhaps permanent injury.

More generally, if you're going for true-blue self-defense any sporting-oriented art is good but not great - you'll do well against an untrained assailant, which is likely a good chunk of the threat model.

It is the confidence in being able to defend oneself that leads one to avoid having fights intitiated with them by virtue of how they comport themselves in a relaxed unthreatened manner.

To top it off, training in something like muay thai will get the kid a great headstart on leading a fit lifestyle. Never done an aerobic activity more tiring or sweat inducing.