In reply to tony forster: It could be down to the role that you are being asked to do and/or where they are going to hold the expedition. The Supervisor is responsible for the team whilst they are trained, practiced and qualify. (not nessicarily the same person on each session)
On qualifying the supervisors role is to remotely supervise the team. Not by going to the cafe. Working out where the team may go wrong and be in a position to circumvent it.
From the recently released DofE Expedition Guide, page 18 "Whilst the DofE does not insist on specific qualifications, it is strongly recommended that suitable national qualifications for the choosen mode of travel are obtained"
Silver can be completed in Wild Country so an ML may be an advantage. ML dosen't solve all the problems as there are plenty of people with the appropriate experience who could supervise an expedition without an ML.
However the school will conduct a Risk Assessment and one of the mitigating solutions could be to use supervisors with ML as DofE are "recommending" that to be best practice.
On the other hand to have a keen, ML trained, volunteer parent to assist with an expedition I don't understand why they haven't snatched your hand off. Good logbook time in my opinion. Second guessing where a group of teenagers could go wrong and be in a position to able to locate them and to help them work out where they should be. Saves a lot of time and stress.
I hope that you enjoy the experience.