What it teaches
Copy, paste, cut, undo, select all, save, print and find. Eight, because a list of fifty is a list nobody learns, and these eight cover nearly everything a child does on a computer at this age.
Undo is the one worth knowing first. A child who knows that Ctrl+Z takes back the last thing they did is much braver about trying things.
How it works
The shortcut appears and the answers say what it does. Both key names are shown — Ctrl and Cmd — because a child will meet one at home and the other at school, and being told only one is how you end up believing the other machine is broken.
What to look for
Use them in front of the child rather than explaining them. A shortcut seen three times is learned; a shortcut described once is not.