Tell them not to wrap the gifts...Just put them in gift bags.
Send them two paged invitation card and on second page, Place a note saying: "Best things in life comes open, so be open and let it show", And place a smilie under word open. Also place pictures of gifts just tied by ribbon and not by gift wrap on all the pages of card, this will also give them idea of bringing gift unwrapped. Do, take a look at another situation in this blurtit's question.