I popped over to see Sandy and found that she had done this with her first grader. ZBoy is in third, so I expected some interesting answers (actually, when you ask kids, ALL their answers are interesting). He's just old enough to have heard some of these, but the ones he's not familiar with show some real insight. Z's completions are in italics.
1. Don't change horses... before you die.
2. Strike while the... monster is farting. (huh?)
3. It's always darkest before... cats.
4. Never understimate the power of... the Force. (gee, Star Wars fan you think?)
5. No news is... new news.
6. A miss is as good as a... win.
7. You can't teach an old dog new... tricks.
8. If you lie down with dogs, you'll... drool.
9. Love all, trust... none.
10. The pen is mightier than the... pencil.
11. An idle mind is... stupid.
12. Happy is the bride who... cried. (?)
13. A penny saved is... a penny earned.
14. Don't put off until tomorrow what... you believe is right.
15. Laugh and the world laughs with you, cry and... the world cries with you.
16. There are none so blind as... those who see.
17. Children should be seen and not... heard.
18. You get out of something only what you... want.
19. When the blind lead the blind... the blind face the facts.
20. Better late than... sorry.
21. Where there's smoke, there's... a fire.
Try this with your own kid. Their answers might amaze you!