Depends on the cardstock used. I'd imagine that plastic coated cardboard won't be very good for you. If you really have to do this, can you cheat a little and only eat a small patience sized deck, or perhaps those cardboardy ones out of Christmas Crackers? Could you use food colouring to mark AS,2S, 3S and so on making a full 52 'card'deck on some small biscuits?
Eating a full deck of cards is not going to be a good idea. While paper itself is "edible" in the sense that you'll just pass it through your system. Playing cards are a completely different matter and could end up damaging your intestines and other organs in the process.
I like the idea of eating crackers that are colored or made to look like playing cards better. It's much safer and won't end up causing you to have to go to the Emergency room.
If you're OK eating a sheet of A4, just write on it (with food colouring!), "A full deck of cards" and put it in an empty card box. When you're called on to make good your promise, produce and proudly display the card box, open it and remove the sheet of paper, show the words then eat it sharpish! You may need to leave town in a hurry right afterwards....