Here are a few ideas and resources for teaching your children about missions. Jesus commanded us to go and make disciples and I think it is important for children to learn at an early age to be a part of what God is doing in the world, that He made us for that very purpose.
Child Evangelism Fellowship (CEF) - This organization publishes material perfect for teaching about missionaries. There are picture stories, often in 5 or 6 parts, that work well for Vacation Bible school or Sunday school type settings. Our church used the one on Hudson Taylor this summer for our Mission Club.
Wycliffe for Kids - some downloadable curriculum to do with your kids or church group. This summer I used "Bread for All" lesson and "Phonics Lesson"
Some books that our on our reading list for the future include some of YWAM publishing, a series of biographies of heros of the faith.
And a book that my husband and I are currently reading: Becoming a World Changing FamilyBecoming World Changing Family