Have you read Animorphs? Those books were bought entirely by people judging them based on their cover. They have kids on them turning into a different animal on each. That just looked cool. If I had had the money for it, I too would have collected them, entirely based on the animals on the cover (I'll take the dog, the hawk, and the dolphin, thanks). 99% of kids who bought them would have done so for the exact same reason.
The actual story is some of the least kid-friendly I've ever heard. It's filled with body horror, cosmic horror, guerilla warfare and espionage with child soldiers, children losing their bodies, children losing their minds through torture, kids having existential crises after witnessing and participating horrific acts of war, countless murders of loved ones, planet scale genocide, etc. It's not for normies. I don't know anyone personally that has actually read the whole series, including myself. I have watched a guy on YouTube go over the plot in summary and... holy fuck. Those books are darker than anything I've read as an adult.