Tell them that they are getting $X from you to use as they see fit - expensive rehearsal dinner, down payment on house, honeymoon. Whatever. But that is your gift to them. My parents did this and we had a really nice down to earth wedding in our beautiful back yard, good catered food, but without all of the expensive trappings that don't really mean anything like engraved napkins. We paid for the whole wedding with the gift from my parents and still had money left to cover the honeymoon, and we did go on a nice, non-bargain, trip.
If they are mature, they'll realize that spending money on being with people they care for, and having experiences traveling, etc, is way more important than balloon arches in your bridal colors and shoes dyed to match.
Have fun!