It hard to believe that a varmit would be so selective. Your really not going to be able to stop them with a sign. I would suggest though a basket with a sign from time to time with extra veggies in it along with a sign saying something like these are for you as we know you must be hungry and if you need more please help us it take a lot of work and time to grow this food. I can't think of why any one would do such a thing if they really didn't need it. You would see more damage and squashed produce if it was just kids being bad. I hope this helps with what could be two problems.

thoughts:
1) sign explaing that it is a teaching/community garden.
2) pictures of the children (no names, or made up first names only) to show who DOES work there.
3) is there a fence around it?
4) friends put up "poison" signs all around their fruit trees and it helped.

Good Luck.....AMS

Dusting with chile peppers helps..

You need to remember to use gloves and wash well..

Two silly thoughts...
A border/hedge of really prickly plants?
A sign with cartoons, showing a face puking and something indicating the plants have been sprayed? (Your kids can be told it's not really true.)

Here's a thought -- post hazardous waste/materials signs; the ones with the 3 triangles. Tell about the garden being a government testing zone -- about the (nuclear) energy commission, or whoever, trying to find out what nuclear waste/isotopes, whatever, does to food crops. Tee hee hee.

Me, I would wait out a few nights in a vain hope of catching them lol!
"Stop! Thief!"
LOL
Josie