Best book for teaching post-beginner kid
Possibly:
Winning Chess by Irving Chernev and Fred Reinfeld (1949)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf

Another possibility, although it isn't a straight tactics book (it's some tactics and some strategy, geared towards kids) is Winning Chess Strategy for Kids, by Jeff Coakley. Jeff has actually written four books, and probably only the Winning Chess Exercises for Kids is too advanced for him.
Author's site: http://coakleychess.com/
Review of Winning Chess Strategy for Kids (Don't order from ChessCafe, they're essentially out of business): https://web.archive.org/web/20140708094112/http://www.chesscafe.com/text/review332.pdf
If this one looks a bit too hard, you might consider Jeff's other two volumes on Winning Chess Puzzles for Kids.
More reviews of Coakley's books:
https://web.archive.org/web/20140708110137/http://www.chesscafe.com/text/review570.pdf
https://web.archive.org/web/20140708234342/http://www.chesscafe.com/text/review754.pdf
My son is not quite yet 10. He knows the moves of chess, and if he doesn't play too quickly, can make some very good moves. We've worked through John Bain's wonderful "Chess Tactics for Students" together, and he really liked that book. He wants another book similar to this one to do either checkmates, or more tactics. He's not at the stage yet where he can concentrate on a lot of text or variations, and I'm not really too worried about him not playing "correctly"--I just want him to have fun at this point. Does anyone have any recommendations for which book I should get him next? Thanks in advance for any suggestions!