I was also quite impressed by a demo I had at the Family History Guide booth. They have a children tab on their site that contains many helpful links. And for the teenagers and bigger kids, their training resources for FamilySearch seem to be quite useful.
One of the finalists in this morning's innovator showdown made me think of the kids: Twile (and congratulations to them for winning 3rd place and People's Choice) is an online timeline for your family history and family present. It looks to me to be a great, very visual way to teach your children about the family they belong to.