How about some headphones that actually limit the volume in the ear pieces?  These are ideal for kids who might turn up the volume a little too much in the classroom, etc.  Another option is a headphone that does not limit the volume but notifies the teacher or parent when the child has pushed the sound level a little high; the built-in light turns from green to red.

At Encore Data Products we offer a selection of headphones that fit the above descriptions.  Products range from "tot size" up to adult versions.