ARboreal Mammals?

I can't quite understand why my children get so excited about their Advanced Reading(AR) goal assignments. Every year their teachers assign a points goal for each student to reach based on their individual reading test scores. Every book in the school library is each worth a certain number of points if they read it and pass a short test on it.
Orangeboy and his sister are very competitive about their AR goals. They can't wait to get their goal assignments near the beginning of each school year. The day the goals are assigned they get to go to the school library and check out a book or two to get started. As soon as the kids are out of school they start comparing their goals and their books. It's pretty much guaranteed that if one of them happens to have a goal that's a couple of points less than the other the one with the lower goal is going to do everything to meet it first.

"I had to get thirty-TWO points, you only had to get thirty!"
"Yeah, but I got my goal first. You read slower."

Anything to beat the other. They just never stop. And today was the day. They received their goals. Sister's goal was higher. This year they are in the same class so they didn't even have to wait until after school to start competing. They started as soon as their class hit the library. Sister checked out a book in the Artemis Fowl series, a fantasy fiction book based around a twelve-year-old main character, that was over 300 pages. Orangeboy checked out "The Amber Spyglass" by Philip Pullman. It's a New York Times bestseller that is 465 pages long. Obviously somebody goofed. They go to a school that houses only fourth and fifth graders. Why would the library have this book? It is not youth lit. On the cover a reviewer credits the author with creating "a world filled with strange divinations and wordplays."
Wordplays? Right, Orangeboy is going to understand wordplays. The reviews also describe it as "powerfully emotional" with much "sensuous description" and with a conclusion that is "heartbreaking but fitting".

I read the first paragraph and asked Orangeboy if he knew the meaning of some of the words there.
"Do you know what rhododendrons are?"
"No."
"Do you know what linnets are?"
"No."
"I think they are a kind of bird. Do know what aboreal mammals are?"
"No."

Orangeboy claims that he did not choose the book on the basis of the number of pages and points, but because of the word "spyglass" in the title. That makes sense. He likes telescopes, binoculars, magnifying glasses and such, so he probably thought it was actually about a spyglass and didn't guess it was about a fantasy world of witches, angels and daemons (yes with an a) - whatever those are.

I think the book is a bit over the capabilities of a ten-year-old, to say the least. And I especially believe a numbers loving, literal-minded ten year old boy is going to have a hard time plowing through 465 pages of lofty fantasy literature - even if he did write that three page story about clouds last year. He'll have to take it back and check out something else, I'm afraid. He was pretty disappointed. The book is marked as being worth 26 points. That would cover most of his AR goal. He'd beat his sister for sure, if by chance he could understand it well enough to pass the comprehension test. Sister's Artemis Fowl book is only worth 12 points.
Maybe I should let him try it. He has certainly surprised me before.