We do not exchange presents with adults beyond our parents. This includes kids over the age of 18 (ie, when it degrades into everyone just swapping itunes cards back and forth).
The kids have never noticed, so far as I can tell. They are too into the presents

that they are opening.
If the kids ask, I guess you could say something like "we wanted to give you MORE presents!"????
Or maybe the adults could do a secret santa exchange, if you want to ease into it.
To ease our generation out of the gift giving we collected money one year for a donation in GP/GGP's names. There was just a card.