I worry about the kids in BJJ matches because their joints can still be very loose and the strain of that kind of sport can do long term damage that they'll regret later in life. Now, when you say "kids" are you meaning like 5 years old and up or teens? I'm all for teens (12 and up) who can handle the knowledge and be responsible learning it. Before age 12, I much prefer them learning TKD or boxing with dummies to learn technique. I know some programs are super careful and make sure that when a joint lock is correct, it doesn't get cranked at all. I just worry about the competition when adrenaline runs high and you don't always realize your own strength or the other person's limits.
My son made this for me: