The great thing about doing this with my girls is that they learn the essential truths of the Scriptures, and it spawns all sorts of questions in their minds. One of my favorite conversations was after going through this set of questions:
17. What was Adam like at creation? He was good.The girls wanted to know what death meant, why they die, if Adam's sin affected them, how so, and on and on it went. Now, they will often say "death came to all people, and to us!" This is also Sydney's favorite part as she likes to make a cutting motion across her neck to illustrate death (forgive me if that's a little gruesome, but with three daughters I have to take whatever boyish stuff I can get!).
18. Did Adam remain good? No, he sinned.
19. What is sin? Disobedience to God's law.
20. What is the penalty for sin? Death.
21. What came of Adam's sin? Death came to all people.
22. Why did Adam's sin affect all men? We all sinned in Adam.
All that to say, try it out with your kids. Check out the links below for a couple different catechisms you could use. My girls are 4 & 6 and I use the first one. It is simpler with shorter answers. One caveat is that I like all the answers except #41, so for now we've skipped it until I come up with a better answer. The Westminster Catechism is great, but may be a little lengthy for most people (even though it's "shorter").
Update: Another Catechism Link: