Is your principal going to back you? I would go to them first if
they are the type that is supportive. Did you write an incident
report, letting them know that this mother's kid was misbehaving? I
would suggest that if this mother has a problem with her kid in the
darkroom, have her buy her daughter goggles, and she can wear them.
Make the daughter sign a contract that she will always wear the
goggles when in the darkroom. Maybe even suggest that to the girl
before the mother makes too big a stink. I have a feeling the
daughter will push the mother to drop it then.
I have had so many students in photo and never had a problem with
this. I've even had multi-handicapped who did fine. I have kids who
get credit to be a lab assistant, they mix every chem but the stop
bath. They where protection from breathing the chems. and I've never
had an issue with that either. They actually help keep the other
kids in line, especially about wasting chemistry or making a mess.