What you need:
1. Air hardening clay (purchase here)
2. Items around the house ( toy animals, plants, seeds)
Step by step:
Knead the clay until smooth. Then roll it until it is flat. Imprint plastic animals in the clay. You may substitute plastic animals with plants, seeds, or even animal footprints. Let them dry. It will take a day or two depending on the thickness of the model.
Paint the fossils to look more real or attractive!
Here are the children's finished products.
Final result after a bit of 'touch up' by mommy. Brush a layer of varnish for glossy outcome.