We remembered how fun it was, so Claire and a friend made some more yesterday.
You need new terra cotta pots (unglazed), Patio Paint (or your regional equivalent), and a marker that will work on pots that go outside. (We found all three of these things together at Michael's.)
You can buy individual containers of Patio Paint, or a little sampler of many colors, like this.
The kids just dip their thumb (or finger, if you like skinnier bugs) in a color, then press it onto the pot to make the bug's body. For bugs that clearly show the child's fingerprint or thumbprint (the lines and ridges of the print, which make it more personal), have them press their paint-covered thumb onto a bit of paper towel to remove some of the paint before they press it to the pot. (If they go straight from paint to pot, you will get solid oval bugs without the fingerprint itself.)
When the bugs have dried, the kids can embellish them with the marker. (The paint and marker are permanent, so wear a smock or old t-shirt.)
You need to let the whole thing cure for about 72 hours before you stick a plant in it or get it wet.
Claire's pot from preschool has lasted outside for almost four years without aging. (The pot itself has weathered, but the paint and marker have not.)