This is a great idea my friend Simon used in a service last week. It would work really well with 7-11s and is very tactile and visual!
You will need a large world map (you can get these quite cheaply in some cheap book shops at the moment), scissors, pens.
Before praying, cut the map into large pieces- enough for everyone to have a piece of their own.
Give each child a piece of the map and get them to hold it in their hands. Talk about how the world is damaged and destroyed by things that we do- environment, war, pollution. Ask each child to pray for the part of the world they are holding. They can pray silently, out loud or write/ draw their prayers on the map piece.
Now ask God to heal the world and bring the prayers to Him by reassembling the pieces of the map like a giant jigsaw puzzle. Thank God for the world He has made and ask Him to bless it and us.