First point, I'm actually impressed by the responses in this thread so far. Well, not because they have all been wonderful or constructive, but what I mean is that I'm betting there are many of us who are members who have suffered bad child abuse as kids, and no one has jumped on the OP with "you think, that is bad, well, I dealt with x, y, and z so you don't have a clue what real child abuse is!" Possibly because of Umbran's warning above, but either way, a lot of folks deserve credit for not going that route because it really wouldn't lead to a constructive conversation.
Ok, now that that's out of the way, second point. I'm not an expert, but I am a child advocate and have worked with abuse victims for decades. When my parents got divorced when I was 12, my mom went to work for the county DA in the juvenile dept (she retired after 30 years a few years ago), so I also have that experience and her as a resource on the topic. Child abuse has an actual definition on a federal level, and what happened isn't close to child abuse. Nothing in what the OP described has the parents do an action, or due to lack of action, put the kids in imminent danger. States have their own definitions of what abuse is, but I'm pretty sure no state would have that fall under abuse either in my experience. I've seen actual physical and emotional abuse not be "officially" considered abuse by the state before because parental rights is still a big thing. Preventing playing D&D wouldn't come close to falling under that definition.
That said, the best advice in this thread is those who have said to drop it. Here are some analogies to help explain:
I also occasionally do firearms instruction. A few months ago one of my son's friends (18 years old, but lived at home still) asked if he could come out to the range and learn how to shoot. I told him I would need to make sure his parents were OK with that. Even at 18 since he lived with them still. Certainly if he was younger than 18. How would you feel if your teen kid just went out to the range with an adult and started shooting firearms without your consent? A lot of people would be upset, even though I could ensure total safety.
How would you feel if your kid went over to a person's house and they started preaching a religion to them, and encouraged the kid to perform in the rituals of that religion against your wishes? I'm sure you'd be pretty upset that they were doing that, and probably would have alarm bells of a cult or danger if it was a grown adult bringing children into these sessions.
Point is, you don't ever want to rock the boat of a family dynamic, even if you personally don't agree with it or don't see any danger. That way leads to big trouble for you, including but not limited to to restraining orders, arrests, and civil suits. Not to mention the horrible position you're putting the kids into. You're literally setting them up for failure, because they have to live with their parents, you don't. So each time you encourage them to break the rules, they are the ones who have to suffer the consequences, not you.
OK, I'll step off my pedestal now.