If I recall correctly, Judo doesn't use many joint locks either, right?
I have no idea if they're taught in a children's Judo curriculum, but the joint locks popular in MMA (like arm bars) are all definitely derived from Judo (which Brazilian Jiu Jitsu was derived from).
When I was at Seattle Dojo (the first Judo dojo in the states) trying out a class, there were two girls around the age of 13 giggling as they were practicing collar chokes with an instructor. It's part of the sport, and IMO no different than that aspect of MMA training. Why would it okay in sport Judo but not elsewhere?