i would be concerned with pestisides, insectisides.... and fertilizer to mention a few.. Even if you are not using it yourself your neighbors could be... birds bring it too.. the only safe worms are the ones you raise yourself.. and although you may get away with it and everything works out once...sooner or later you can run into trouble.. and the risk is not worth losing your ray babies....Its easy to cultivate them yourself.. that way you know they are 100% safe.. have the kids share the project..
cultivating Earthworms
Earthworms are some of the most complete sources of food in nature. These worms are full of vitamins, roughage and calcium. They have rich supplies of proteins too. Fish that are fed on earthworms grow at a very fast rate. Large fish like the Cichlids will eat the adult earthworm in whole, while medium size fish will eat worms that are an inch or less in length. The very small fish species will need the worms chopped or shredded.
Earthworms are blind, but light sensitive. They will burrow into the soil to avoid light. They are also temperature sensitive, and will drown if their home is flooded. Earthworms prefer a rich, clay free soil. You can feed them with decaying vegetables and eggshells. Do not over-feed the worms, as this will turn the soil sour. The simplest way to culture a small quantity of worms is to gather some very heavy sacking on the ground.{i would use a large wooden box} This should be in a shady spot. The area should be kept moist. Some scraps of vegetables should be placed in between the sack and the ground to encourage the growth of the worms. When you need the worms, just lift up the sack and remove the worms that cling to the sack.