A plaster mold, made from plastic, is a good way to do a relief project with younger children. These molds are fairly inexpensive and can be reused many times. Because a pre-made mold needs no special skill or artistic input, it is easy to mix up a batch of plaster, pour it into molds and have the relief shapes ready for children to paint once the shapes have set.
Creating a clay mold is a good project for children who are a bit older. Roll out slabs of clay, then allow children to press objects into the clay to make a pattern. You can also sculpt patterns onto the wet clay. Form a low "wall" around the edge of the pattern, then carefully pour in a little plaster. This makes a unique relief that requires a little more time and skill than a plastic mold relief.
Create thin slabs of plaster. A cardboard box could easily be used for a mold for slabs. Draw on the slabs with a pencil, then use carving tools, such as knives or small chisels, to carve out the pattern. This process is somewhat time-consuming but allows a child to essentially draw or etch onto the piece of plaster, making it possible to create a scene or image. Wear goggles during this project. Because it involves sharp tools, this is a project that is suitable for older children.
Plaster is generally a safe material and is very safe once set. However, it is extremely important to monitor children around wet plaster. A child may understand that plaster is used in casts and think that it is safe to put on his or her body, but a physician does not use plaster in this manner. Plaster becomes very hot while it sets and can cause burns, so it is important to monitor children to ensure that they are not playing with wet plaster. If you are worried about safety during a project, let children pick out or create molds and pour the plaster yourself.