I was searching for card stock at home today and was having trouble finding anything sufficent. I was digging through my walk in closet and found clothes boxes.  Like the ones JCPennys wraps clothes in at Christmas time. Ha, so anyway I created some sick stencils just using that. If you unhinge the corners and flip em up they act as a cool "tray" and catches unwanted paint from transfering.... I don't know if that really helps but it's one of those items lying around the house that kids could easily access. 
something else that works pretty well I find:
got any old Wham! records, christmas albums from the chipmonks, or country albums that you think you
can live without? the sleeves work great for stencils and fold out from 13" by 13" to 26" by 13" so
depenting on the size of your bag, they can fit in there.
You can also go to garage sales/used record stores, they sometimes have free boxes.
contact paper, wax paper, coffee filters, take pictures to a place to be laminated,cut into lamination