Once we let them all stamp their hands in green paint and layered them upside down and made a 'christmas tree' out of them, maybe even making some 'reindeer' with some of the little hands. Although this idea would really only work if the auction were around Christmas
I also saw one last year at a school auction (it was done on a scrapbook) and it was just their thumbprints. Each thumbprint was made in to a ladybug and then the teacher had used the same black paint marker (that was used to dot the red thumbprints) to make some 'flight dots' behind each lady bug where they were flying around. Then blended in to each flight pattern/dots were the childs name that the thumbprint belonged to...it was too cute.
You could also do a tree on it and then let their hands be the leaves falling off the tree in fall. Each hand be a different fall color at the base of the tree. You could add a pumpkin or something else fall looking to the apron, maybe a cute pumpkin timer, or a basket with the 'fall apron' and add some other fall things like pumpkin pie spice, hot cocoa or spiced cider mix...etc.
As far as sayings, you could put something like "hands down, these kids are great".
Helping hands in the kitchen
Hands on help
give the chef a hand
I can't think of anything really snappy though...good luck

It sounds cute and I'm sure it'll be a big hit!