Quick and easy! Have your child glue one O on top, and one on the bottom, between a horizontal line, to make a lightswitch.
Then write the words "On" and "Off" by adding the letters. A trick I picked up volunteering in my sons' kindergarten class, is to use highlighter for children to trace overtop of. That way you can see their pencil lines, and it's a bit easier than the traditional 'connect the dots'.
Do you remember when your child first learned to turn the light on or off? I know my boys learned 'off' first because it's lower and easier to reach. I have a memory of my oldest son running around the house, constantly turning all of the lights off (just because he could!).
Just a few inches taller and he could really make magic, when he learned to press both On and Off on the lightswitch.
Now I'm constantly reminding them to turn the lights off… and don't even get me started on the endless intrigue of the dimmer switch… why did our contracter install a whole panel of them within child's reach in the basement again?