Thanks for the tag.
Like Pwn said, I coached martial arts for close to a decade. My background is in hapkido (basically the Korean version of Japanese ju-jitsu combined with TKD), BJJ, muay thai, boxing, plus silat and some judo. I’ve taught all ages — kids, youth, adults — but I was the head instructor for my school’s kids and youth program.
Some semantics: any art that involves fighting is a martial art. Boxing, wrestling, judo — they’re martial arts, not just the “fancy” ones. I really don’t think it matters at a young age which art kids try, as long as they enjoy it. Until they get to be teens and have a better mental grasp on what fighting actually entails, the specific style doesn’t matter.
All that to say, if I was pressed, I would argue that boxing and wrestling are possibly the two simplest and best arts for kids to start with. Why? Because they’re simple and effective. Kids can only retain so much information and don’t have the greatest body awareness. The subtleties of even something like kickboxing can get lost on them. Something like forms in TKD? It just becomes busywork for them, as opposed to an effective visualization exercise (sidenote: if anyone wants to know why forms are actually very important, I’m happy to chat about it).
My personal recommendation is that kids start out with boxing or wrestling, then as they get older and figure out what they enjoy, branch off to specialize a bit more. If they like boxing, head into muay thai/TKD/karate territory, and if they like wrestling, go to BJJ/judo/hapkido. In fact, this advice applies to ANYONE of any age who wants to start martial arts. Just like lifting, start with a base, then specialize.
Final note: I agree with Pwn; if someone wants to become a monster in a fairly short period of time, a combination of boxing and wrestling OR judo, plus lifting, will make you a scary motherfucker in a handful of years. Even with my extensive background, when this pandemic is over, I’m sticking with those two/three.