Collect milk jugs and cut them at the line at the bottom. Save them until you have enough since they are a one use mold.
Get: QuickCrete, glass beads or findings, or special rocks, broken ceramic dishes to inset.
You'll need plastic trashbags or something to drape over the kids, open air area for concrete dust, newspapers for the surface, bucket or container for the beads, findings and stuff to inset. Don't use organic material such as leaves, as they may rot. If you only want the imprint, that would work.
Mix it to the right consistency. Not too runny. It will set up within an hour, so mix as you are ready to make them. Best if they dry slowly - cooler, out of the sun, to prevent cracking.