Maybe some sort of book that you think will be special to them. I've received a lot of books over the years, and they have been treasured a hell of a lot more than anything else I own (please don't buy the Bible). I have a dictionary that my grandfather gave me years ago. I know it's just a dictionary, and I never actually use it, but part of the memory of him is in that book.
I was a page boy and my sister was the flower girl for an aunt and uncle when we were a similar age to your children. We have always just remembered it, despite it being a fairly long time ago. As I remember it, she was far more interested in looking at people and asking me questions than she was in the actual wedding, hah. Very typical!