How can The Secret Garden's language not be dark? It's about two love-starved kids who become emotionally and even physically abusive to the people around them because they feel so sorry for themselves. Sure, there's Magic and friend making and gardening and they get a lot better, but the set-up of both Mary and Colin's coming-of-age is surprisingly serious. What's more, the novel never softens its prose to avoid the harsh truths of its main characters' weaknesses. Check out this passage from Chapter 16, for example:
Because [Mary] was the stronger of the two, she was beginning to get the better of [Colin]. The truth was that he had never had a fight with any one like himself in his life and, upon the whole, it was rather good for him, though neighter he nor Mary knew anything about that. He turned his head on his pillow and shut his eyes and a big tear was squeezed out and ran down his check. He was beginning to feel pathetic and sorry for himself—not for any one else. (16. 40)
Colin is our hero, guys, but we still see him in the middle of a seriously bratty argument over whether or not Mary is allowed to have other friends (in this case, Dickon). And as he's falling into his jealous rage, "a big tear was squeezed out and ran down his cheek." That's classic, non-heroic emotional manipulation right there: this tear is supposed to guilt Mary into ending the fight and giving in. Of course, Mary doesn't back down, but that doesn't change the fact that we see Colin in a really unflattering light here.
At the same time, it's precisely because we know that we are getting a blunt and straightforward portrayal of Colin's early weaknesses that we can trust his later improvement. This fight, as ugly and selfish as it appears, will be "rather good for him," the passage promises. It's because of the novel's dark descriptions of its main characters, their self-absorption, and their damaging behavior that we continue to feel hope that, by the end of The Secret Garden, they will be able genuinely to sort out all of these bad feelings—and they do. Phew.