I just finished reading The Invisible Heart: An Economic Romance by Russ Roberts, an economist at GMU and blogger at Cafe Hayek. I'm now reading it to my 9 year old daughter who has definitely enjoyed the first three chapters and I have to heartily recommend the book for the purpose of introducing children to libertarian economics. It's an easy read, with lots of examples of economic issues. It's remarkably humerous given the subject matter, so it keeps the attention of those who generally fall asleep at the mere mention of economics. The plot has an interesting and surprising twist. The romance part is a little lame, but at least it's not steamy, so I can read it to my daughter without any worries.
All in all, I consider this obscure little book a real gem, especially for those who want to introduce younger folk to economics.