Book for Boy Entering High School with Cognitive Delay (and siblings)

So during the school year at a weekly church activity, I work one-on-one with a boy with pretty significant cognitive delays. His whole family has become near and dear to me. At some point this summer, I need to go to their house and present the awards to him and his sister form the program (they had to miss awards because he was sick) and I'd also like to get them each a book.

The guy I work with is going to be a freshman in high school, but still loves Bluey and other things that would be considered "younger." (Peppa pig, Elmo, etc.) He can read words quite well (in fact, I'd almost say that part is close to grade level), but comprehension and longer term recall is low. I'd like to get him a book that's above Bluey level, but still appropriate. Something that doesn't look/feel as "kid-ish" but he'd still enjoy.

Also looking for suggestions for his sister who's going into 7th grade. I think I'm going to get his youngest sibling (entering kindergarten) "We Don't Eat our Classmates" and read it to all of them when I go over there. That's the current plan, at least, subject to changeB