I LOVED this song when I was in Primary!!! My mom is going to be singing this with her primary children this week to go along with Septembers theme (The Gospel Will be Preached in All the World).
It's called 'Go on a Mission" words and music by Pat Kelsey Graham. It can be found in the Aug 1986 Friend or here http://lds.org/friend/1986/08/go-on-a-mission?lang=eng. It has a catchy tune and hopefully your children will LOVE it, too!!!