If it was my kids going, pictures of them having fun, meeting characters, riding rides, etc. would be my first choice by a long shot. If the CMs take an especially nice picture of the kids with a character or maybe with a good background (like the Castle or a favorite spot), I'd love to have a copy of that along with copies of whatever snaps your family takes. If you think the parents would like a Disney frame, that'd be a nice way to present the photo. If not, a regular frame would work just as well.
If they collect something specific, you could add to that collection.