I recommend you find a copy of Lenchner's book Creative Problem Solving in School Mathematics (2nd ed.). Lenchner ran math olympiads for late elementary school

I recommend you find a copy of Lenchner's book Creative Problem Solving in School Mathematics (2nd ed.). Lenchner ran math olympiads for late elementary school / middle school for decades, and does a good job of covering a wide variety of useful strategies for approaching nontrivial word problems (i.e. real math problems, not just arithmetic drills). But you don't have to be interested in contests per se to get a lot from the book. In my opinion that's going to be more pedagogically valuable and more fun than Trachtenberg. Here's an Internet Archive scan https://archive.org/details/creativeproblems0000lenc https://archive.org/details/creativeproblems0000lenc Your kid would also probably really enjoy The Number Devil, a breezy novel about a kid who gets trapped in his dreams with a devil who won't stop telling him about elementary number theory, using various dream-world props. If you want more problems after that, there are a lot of great puzzles in Kordemsky's The Moscow Puzzles https://archive.org/details/moscowpuzzles3590000kord_m9a0 https://archive.org/details/moscowpuzzles3590000kord_m9a0, or you could take a look at Fomin, Genkin, & Itenberg's Mathematical Circles: Russian Experience which your kid could probably just about handle. There are also some great puzzles and games explained at the website of the Julia Robinson Math Festival, https://jrmf.org https://jrmf.org which can be played with fairly basic easily available materials.