Jennifer:
I wouldn't go out there tonight. I would ensure that my children and pets do NOT go to the back tomorrow either.
It depends upon what is UNDER your deck. I would put on some shoes and walk around IN A CIRCLE in the daylight with gloves on and pick it up. If its dirt or concrete - it will be easy. If it's mulch or grass - it will be tedious. There really is no easy way to do it that I know of.
You walk in a circle so you can catch the sun glinting off of it. That's the easiest way to spot it - walking around and around. Chances are, if you walk in a circle when you find one piece you will most likely find another.
Bottom line? Gloves and GOOD shoes - not flip flops with a box not a bag (you don't want the glass to cut the bag and end up with another mess).
GOOD LUCK!!