Here is something I've considered. I only have the first volume, though, and am not sure if it gets at the mathematics the way I'd like. One thing I can say is that the book is very funny to my 5 year old.

Another resource that I've heard is good but have not yet tried is Beast Academy, which was produced by the Art of Problem Solving. These present mathematics topics in a graphic novel format. Children purportedly have a hard time putting them down.