Find a straight-on view of a photo of a bat head.
Print it or cut it out and make it the size of about 8 x 10. (maybe someone you know has a printer or graphics program)
Back it with construction paper or cardboard.
Cut out eyes nose and mouth fitted to the location of the person who will be wearing it.
Attach an elastic or rubber band and staple it to the back of the mask.
Put it on the child and see what needs adjusting or trimming so they can breathe, talk and see properly