How about if you have a tshirt made that says local hero and have kids sign it then if you have a budget give them a gift card to the local coffee shop of choice.
I agree with the plaque idea, but if you want to give them something in hand at the actual event you could get an award made up at a local engraving store.
Last year, we had the local firefighters at an event and we took a great group photo of the kids with their hero The local trophy shop made a beautiful "Thank You" plaque with the picture. It was inexpensive and great. It's hanging on the wall in the firehouse. A few weeks ago, my son's scout den toured the firehouse and was so proud to see it there!
Our school is doing a Book Fair/ Ice Cream Social/ Super hero night and wanted to know if anyone had any simple gift ideas to give our local Super Heros..a police officer, fire fighter and a vet. I got nothin'.