Reddit mentions of The Toon Treasury of Classic Children's Comics

The DC Showcase collections are a good source of cheap reprints of suitable silver age comics such as Batman and Superman, and also - as per the Goosebumps connections - some of their 60s/70s supernatural titles like House of Mystery, Ghosts and The Witching Hour. Here's the Ghosts volume, which is 512 pages for $12 - great value. I wouldn't imagine any of the Showcase volumes would be inappropriate (I'm in the UK, but I'm thinking grade 3 is around 9 years old?). Marvel have their cheap paperback equivalents, and I'm sure classic Spider-Man would be a hit.

I also think you can't go wrong with Carl Barks' Donald Duck strips, and Fantagraphics have just launched the first in a new series of reprints of those, starting with the excellent Lost in the Andes - if you don't have access to that your local library might have some Barks stuff in a Duck Tales collection or the like. These stories are wonderfully told and genuinely funny.

Returning to the supernatural angle, at around age 9 I was fascinated by a Little Archie digest I picked up. Little Archie stories usually revolve around spooky plots and - in the 70s, at least - they were imaginative and well done. Older Archie stories in general I think are a good bet, but Little Archie brings the mind-bending horror (I still reread a great story about a mad artist trapping people in his paintings for decades).

You might also consider the recent collection of Dick Briefer's Frankenstein or Bob Powell's Terror. These are all collected editions - I'm afraid I don't have much info on current releases that might be suitable - but may be worth checking your library for.

And finally here are my top (and lavish) selections for young readers - Golden Collection of Krazy Kool Klassic Kids' Komics and The TOON Treasury of Classic Children's Comics - both are excellent collections of classic strips.

And really finally, as the last link just reminded me of it, check out Little Lit - Art Spiegelman's collection with modern creators doing new comics aimed at kids, with quality results. That's a link to a compilation of the best, but there are themed single editions.