Kingmaker does have a couple of things you may need to modify, depending on how mature your 10 year old is.
Brother's Grimm-like Fey:
There are a couple of dark fey running around in Book 2 that drink blood and collect severed heads, but could be swapped out easily enough to change the tone. There's also fey dealings in Book 6, but I don't remember anything that creepy.
Big Bad of Book 3:
The big bad undead (powered down) lich at the end of Book 3 eats brains to absorb the memories of its victims, and has a morbid 'dining room' where all of the leaders of one village are sitting down as if to eat, but with their heads cracked open and brains gone. There's also special soul jars storing almost a whole rest of the village, so it'd be easy to make all of his victims stuck in the jars. He's also a worshiper of one of the Horsemen (with a creepy altar), and has multiple undead brain eating followers, but I think the squick level can be downplayed fairly easily.
Kingdom Matters:
There is at least one rival nation in the Kingmaker books that acts friendly while simultaneously trying to stab the PC's kingdom in the back. You even interact with this kingdom at a tournament, where they're surreptitiously trying to sabotage the PC kingdom during the events Of course, it could also be good training for high school....