From what I remember, where the movie has a longer guy discover a powerful dragon and with that starts a classic hero arc, the books are about a guy who gets very unlucky and is generally not very impressive, but still manages again and again to sort of stumble through adventures. In the books every kid gets a dragon to train, as a kind of rite of initiation. The main character only manages to get a tiny, rather pathetic dragon with literally no teeth (hence the name toothless). And he and his dragon both stay kind of pathetic as big events happen around him, but he keeps managing to somehow save the day, but of course no one realises what he has done and thinks it a fluke that he survived at all.

I read only the first couple books, so maybe it changes after that, or maybe I’m misremembering things. I really liked the story, but it’s certainly quite different to the fairly traditional hero arc of the movie. If your kids loves the movie there’s a good chance the book would disappoint them.