Your child has decided they are a superhero. Good news: so is a whole shelf, and it runs from three years old to thirteen. Reach for whichever one fits and ignore the rest until you need them. Pick by two things: how old your child is, and which bit of superheroes they've actually fallen for. Some want the costume and the power. Some want the joke. Some want a real comic with panels to get lost in. The ages here are wide on purpose, and they matter more than on most shelves, so read the age on a book before you buy it.
Superheroes look like one subject and they're really three, which is why this shelf is sorted by appetite rather than by age alone. If what your child loves is the costume and the power, that's SuperHero ABC: twenty-six invented heroes, one per letter, drawn by a man who drew Spider-Man for a living. If what they love is the joke, that's Captain Underpants, and it's worth being clear-eyed that it's barely a superhero book. It's a toilet joke wearing a cape. It has also turned more children into readers than anything else on this list, so it stays. And if what they want is the real thing, an actual comic with panels, that's the other five: Zita, DC Super Hero Girls, Sidekicks, Primer and Ms. Marvel. One thing worth naming, because it happened without anyone setting out to do it. This shelf is broad. DC Super Hero Girls and Primer are led by girls. Zita is a girl in space. Kamala Khan is a Pakistani-American Muslim teenager in Jersey City. For a child who has so far only met Batman, that is the point of the whole thing. The one call to get right is the top end. Ms. Marvel is for eleven and up, and it isn't a near miss for a younger child.