While I don't yet have children of my own, I have more experience with children than you might imagine. I have found most of them to be far more intelligent than they are given credit for, and (while they certainly don't have a full understanding of everything they encounter) they are almost always curious and eager to learn. The trick that has worked well for me is simply to treat them like people; talk to them, answer questions, and allow them to pretty much do what they want. I suppose YMMV, but so far I've had very good results from these techniques.