This is a great idea! However, I don't have anything like this at this time.
I think I would I treat it as a Kudos. As soon as I heard a positive comment from a teen, I would have Kudos from I could fill out and send to the department so the manager could hang it up for the whole department to see. Hopefully this would encourage the staff members to continue the positive interaction.
I handle the challenges as they come and usually with a phone call to the manager. ie. the volunteer is bored and does not feel like they have enough to do...do you have additional tasks that they could complete for you? I find I can usually resolve any problems this way, but if a department continues to have problems, I will reassign the teen.
I also have the teens fill out a survey at the end of their service asking how the department did and share the results with the managers.