My son just made an animal mask that fits over his head by using a balloon.  We blew it up until it was about the size of his head, making the end that tied, the part that would be the snout of the animal, that way length could be achieved.  You can add various things like ears with cardboard covered with the paper mache.  Cut an eye hole but glue black mesh to make it invisible.  Paint the eyes on the side to make them appear in the correct position.  Glue a piece of felt on the back to hang down and cover the childs neck.  Make sure it is the same color as the animal.  When you get done and pop the balloon, make a circle out of cardboard about 1 inch in depth and make it the size of your childs head.  Tape this inside the mask where it will fit snugly onto your child head, like a hat!  Hope this helps.