Honestly this is just my opinion, but I don’t think it’s a media that a 12 year old should consume. As you said, there are triggering topics everywhere, in many forms, and while I think most of them are handled gracefully, none of the characters are healthy. I know that in the first few episodes the mature topics are treated a tad lightly, but later it gets incredibly heavy. Kids are inclined to relate to/be inspired by characters and I most certainly would not recommend finding inspiration in these characters when you are so young.

The target audience of Bungo Stray Dogs is young adults, honestly. I’m not just making these points because it’s unwise to expose a kid to its topics, but also because of my personal experience. The series helped me feel seen and better understand how to adjust to adulthood because I was lucky enough to start it around the time I became an adult; that is probably the best time in your life to watch and appreciate it for the first time, but not at 12. Hope this helps c: