THE CREATURES could be sock puppets. However, this would be a problem when the script asks for these beasties to tickle the old couple (Page 7)

Of course if these are used on a stage (as opposed to the classroom) then the "wall" the operators are hid behind would have to be of a thin material to allow their voices to be heard. Failing that. put a microphone at the back with them, but that won't work very well as their voices will then be coming from a loudspeaker and not apparently from the puppets.