I have a group of kids, one of whom is the eldest at ten and is the responsible one. They're on a quest to save the life of a baby from something that wants to kill it, and also wants to kill the main characters too, right? At the end, he's hanging from a ledge and begs for his life, the eldest picks up a rock and smacks him on the fingers. Is this too cruel? Also, the little evil creature reaches up to try and pull her down when she turns around (she turns around to pick up a rock) but she makes the choice to smack him in the fingers without knowing he was doing this, which is why I'm wondering if it's cruel. Also.....I get this heavy vibe that this scene is very.....hollywood? The creature runs at them from afar intending to spear the girl but gets knocked down by something introduced earlier in the story and ends up hanging on the ledge at their mercy. I get the feel this is going to come off as simply a movie cliche.
No. Kids are mindlessly cruel, very often, and this case your character has a very good reason to do what she does. It's been done before and perhaps you can call it a cliché but it all depends how it's written. There are very few original ideas now, we're all just doing things other authors have done but in a different way. That's okay.
Be true to your character. If they want to smash someone's head, rip a baby apart - let them do it. What may matter more however is genre and target audience. You tone it down for reading matter for children and obviously, if horror is just sub-text, then perhaps be a little sparse with the amount of horror. Otherwise, I'd say let your characters 'have at it'.
There's a scene where someone is hanging from a ledge, I understand why you say it's Hollywood. But to answer your question... No it doesn't seem out of the realm of possibility for a kid even at his age.
Maybe it is very cruel. But the real question is, "is this what you want?" The issue is not whether the child is cruel, but whether you want people to think she is cruel. I think in a scenario where the creature has been trying to kill them throughout the story, throwing him off a ledge is rational.
It is in our nature to be cruel to each other, why don't you think Anarchy works? We are a vicious race.