Carol Laut, our districtís Missions Committee Chairman shared this idea at a workshop on teaching missions to children that I helped with a couple of weeks ago:
Have a number of prayer requests for a missionary written on slips of paper in a can or cup. After teaching about missions and talking about the missionary, pass the cup around and have each child pick a request out of the cup that they will then pray for.
Note: This would be a great idea for missions conference. Invite one of the missionaries to provide the requests and come to class to share about their ministry. Then you can have the children pray for the missionary in person.