How to help a child prepare to one day serve in the temple:
Share your testimony of temple worship. If you're a parent, discuss temple worship during family home evenings and other family settings, formal and informal.
Help children understand that if they keep their baptismal covenants they will be worthy to one day attend the temple.
Take them to a temple dedication, if possible and if they are at least 8 years old.
Sing songs about the temple. Many are suggested in the 2002 Outline for Sharing Time and the Children's Sacrament Meeting Presentation. (These can be obtained at Church distribution centers.)