You will need:
Plaster of Paris
Wire 
Large plastic bowl or a disposable container plus a stick to stir
One Plastic lid or one aluminum pie plate

Instructions:

Mix some Plaster of Paris with water until stiff by creamy in the disposable container.

Pour the plaster into the aluminum pie plate or large plastic lid.

Your child can then make a handprint in the plaster.

Before the plaster dries, punch a hole in it for hanging it up or place some wire in it to use to hang up the handprint.

Let Dry.