This site has a cool video podcast for kids. The questions are exactly the types of things most kids want to know. I think it's interesting, but I wouldn't say that the children will truly understand the concepts. Being limited in time and the kid's current context of knowledge, it answers enough, and truthfully and can kindle additional questions. Check it out for your kids who love to ask questions, or love space, or whose interest is just a tick away from becoming motivation to delve further.