Measure equal parts of sand and plaster of Paris into a bowl. Add water to the mixture slowly, mixing it as you pour it. The consistency will be thick but fluid. Pour one-third of the mixture into a large Styrofoam cup. Center a plastic dinosaur in the cup, pushing it down into the plaster mixture. Pour the rest of the plaster into the cup. The dinosaur should be completely covered. Make as many dinosaur molds as you need. Let them dry for at least two days. Peel the cup off of the molds.
Teach your child to work like an archaeologist by reading or viewing videos. Show her how to take careful notes, show the exact location of a find, and draw everything just like they find it. Encourage him to work carefully, noting everything around the dinosaur.
Turn the earth in a 4-foot square in the backyard using a rototiller. Bury one or more of the dinosaur molds. Vary the depths. Walk over the ground and make sure there is no sign to show where you buried the dinosaurs. Give your child a plastic or wooden spoon and send him out on his first archaeological dig. He should be armed with a notebook, pencil, cuticle stick and paintbrush. Include a digital camera to make the simulation realistic.
Use the cuticle stick and paintbrush and carefully dig away the plaster/sand mixture. Help your child find resources to identify the dinosaur once it has been completely unearthed.