A lot of good advice has been given already, I would recommend reading Richard Dawkin's letter to his (then) 10-year old daughter, you can read it here.
I just read it thanks to the link provided. I think is would be a good base to work with.
I have 3 children all very different from each other. The thing is to plant the base thinking idea then let it take root. Children that young tend to have a short attention span all you need is about 5 minutes because after that they tend to stop listening, no one really likes lectures. Let them know that is ok to question anything or anyone. Then go let them be kids...laugh, play, run, pretend and learn.
That was pretty cool. I'd not seen that before.