Who can be a missionary? Only one who knows the Lord as his own Savior.
Be sure to make this clear to the children you teach. The unsaved child needs to know that he cannot serve God until he has first placed trust in Him for salvation. The saved child needs to know that God will use him to win others to Christ starting today and even down the road as a full-time missionary. This may be an opportunity to let the unbelieving child think more about the importance of receiving Christ and the believer the importance of considering missions as a life work.