Here is a small booklet I made for my own kids, which may be of interest to yours as well.
There are two versions - an adobe acrobat (pdf) file in a regular printing mode (every sheet contains one page), and a "challenging" file, in the shape of a booklet (with two pages in every sheet, which allows for forming a booklet, but requires paying attention when folding and putting the pages together).
The important principle shown is that addition is always addition, no matter what objects we add: tangible objects, numbers (or other abstract objects), and even concepts which meaning we don't understand, or which don't exist in reality.