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.
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.