Cover the child with a blanket and have some cloth towels or paper towels nearby to use during the process.
Children can work in pairs.
Play some relaxing music to make the environment relaxing. Overlap and smooth plaster as you work. The child can decide if they want their eyes, nostrils and mouth covered as this is optional . When all areas that the child wants are covered with plaster and there is a nice firm covered surface, let dry for about 4 minutes or until it is firm enough to the touch to pull it off without the mask falling apart. The mask can be further smoothed out, added to, patched etc.