This is an easy, inexpensive way to pray very visually.
You will need: Chalkboards of any size, cups of water, paintbrushes.
Encourage the children to paint their written or drawn prayers onto the chalkboard with water.
This is particularly good for sorry prayers as the water will dry up and the sorry prayer will fade away as a sign of how God forgives us when we are sorry. For the same reason, it's a good way of helping children to release their prayers to God and visualise them being handed over.