Ooooh, a thread with my name on it!!!!
The plaster of paris masks were very easy (but messy -- you need a supply of old shirts or overalls). The bandages are readily available from art shops (they're also called Modroc). To do masks you need to cut them into little strips about the size of an elastoplast (though you need some smaller ones for fiddly bits like between the nose and mouth). Clearly they need to leave gaps around the eyes and the nostrils.
You need to tie the hair well back, and ideally put a plastic bag (those thin supermarket veggie ones) over the hair, sticking it down with micropore tape around the face. You need to slick vaseline generously over the face, particularly the eyebrows.
Then you just start applying the strips after dipping them in warm water. If it starts drying out, you just smooth it with wet fingers. It takes about three or four layers to get a decent covering -- it's a bit like doing paper mache, only incredibly quick.
You need to leave it on the face for about 10 mins to be dry to the touch, and then get the child to wriggle its face around and you should be able to ease the mask off and leave it to dry properly. Finishing touches can be done later -- we gave the children some extra lengths of bandages to take home with them.
The children did it in pairs, and a complete mask probably takes no more than 20 mins, plus the preparation time and drying time. For 8 or so children it filled a sensible length of time and was done in time for tea (hour and a half, maybe?) We did a couple of trial runs on our kids beforehand, so we knew how it would work. You definitely need at least two adult helpers, and lots of damp cloths for cleaning up afterwards.
The parents thought it was great -- but we had warned them, and the children weren't wearing smart clothes. Also it was summer, so they could run around outside waiting for it to dry.
Just relocated the website I got the original idea from .