this would probably cost too much for 12, but for my 3 volunteers this year, i gave a sand pail with a towel rolled up (about $4 from walmart), a sunscreen chapstick, and a tube of sunscreen. they all really enjoyed it!
we went to a workshop last year where the benefits of jigsaws puzzles were extolled. one of our 5th grade teachers then gave her volunteer parents jigsaw puzzles for the family! (i thought it was an excellent idea; but was bummed because now i can't give them this year!)
i looked quickly for a link to give the benefits, but i give up after about 5 minutes---so you could attach a little card explaining your gift.
*promote collaboration, problem-solving, trigger brain activity, improve memory