Librarian here! Superhero stories from the two major players (DC and Marvel) come in two flavours: there are the ones tailored to children in the form of picture books or early readers. which retell some of the earliest stories of the heroes, from the 40s to 70s, generally. Maybe a handful of movie novelizations. Kids under 10 - heck, kids under 7 - eat them up. And I'm okay with that. The early stories tended to be a bit more wholesome, classic good-triumphs-over-evil type tales with "fantasy violence" (laser beams, maybe some punching.)

THEN you have the modern graphic novels, often compiled as trade paperbacks. I wouldn't let a kid under 13 TOUCH them. They're all too happy to show blood and have heroes acting like villains.