This little tutorial, unearthed on the blog Saltandpeppermoms, should appeal to children ... and parents! :-)
To achieve this you will need:
Step 4: Once your paint is dry, fill the pots. Start by placing the florist foam, place your painted pebbles as you like and fill the pots with small stones to wedge the "cactus" pebbles.
And that's a pretty good creation!
Source : http://saltandpeppermoms.blogspot.fr/2015/04/hand-painted-mini-cactus.html