DH is a blackbelt in TKD, started taking it at 12 from a very serious teacher. DH doesn't believe in taking martial arts for fun. It's serious, or you don't go.
DS has wanted to take a martial art since he was around 3. He started Aikido at a new 6 because DH decided that DS understood this wasn't for fun, and we lucked into a program at the Y run by the Tacoma Aikido Club. Parents do NOT expect a serious class at the Y, and most kids who start don't come back for more than 2 more classes...either Sensei intimidates the kids or the parents, one of the two.
DS stuck it out (probably because he was *determined* to take it AND because he understands the concept of sarcasm, thanks to me, and that's needed with this teacher).
The self-defense he learns isn't "you can get out of the way of adult attackers", but more of the sort of trust your instincts, YELL, MOVE, be difficult if someone grabs you...attract attention, don't just freeze. The teacher has the parents read The Gift of Fear which goes along with the sort of self-defense they talk about in class.
The class is serious; my son has been put in a "time out" (where they stand on the edge of the mat, silently, not taking part in class) 3 times. Once for talking, once for being near a group of talking kids, and once for putting his hands in his pockets (usually an offence that gets you "barked" at, but he's been in it nearly a year and certainly knows better).
He has learned body awareness, precise movements and *control*. Of course, he also takes gymnastics and ballet (and swimming) which help with all of that.
He loves the course, even though (or perhaps because) it's so serious.
And Sensei would want to know if he had ever been aggressive with any moves, and they would talk. DS has used moves *once*, in a bounce house at a company party, when a bigger boy was bouncing into him on purpose and was trying to hit him. We saw DS in there putting his arms up and turning, exactly as taught. It confused the aggressor kid so much, because DS wasn't just taking it, but he wasn't being violent back, he was just deflecting the kid's hands and working to get away.
So find a GOOD class, and I imagine it would be fine. But good classes aren't all that common.
DH spent time in Taiwan and Korea growing up, and got to see Power Rangers before it swept America. In Taiwan and Korea, his relatives and classmates took martial arts but NEVER *played* with it. They did not copy Power Rangers; they understood that martial arts aren't to be played around with. When he came back to America, about a year or so later PR started being shown on TV. And he saw the American kids in his classes go insane about it. They would play at it. They copied the moves, they hurt themselves, they hurt each other. They hadn't a clue, not even the kids who took martial arts (at a place other than where DH took TKD), that it was serious and not a game. And, honestly, DH tends to believe that most martial arts studios are more like play places than serious places, and if we hadn't lucked into this one, DS wouldn't be taking it.
Well, we might have put him into a Tai Chi class (very very slow martial arts blocks, hits, and take-downs, as a book I am reading described it LOL), but nothing more than that.
I took a family class with 3 of my kids the year after my first husband died. I saw it as a way to spend time with them, be active, lose weight, and have fun. Those things we did. None of them really learned discipline or self-defense from it, and we spent a LOT of money. Looking back on it, it was a good experience for us at that time in our lives, however I won't put my 4th child in, and I wouldn't spend that kind of money to do it again.
On a side note, the instructor we had then is now on death row for murdering the parents of 15 special needs kids. Can't even begin to tell you the emotions that evoked.
Oh my goodness, I remember seeing that story in the news.

DH responded with horror when I read your post to him... I'm so sorry.