How far away from your house do they live? I had a similar situation years ago, but the family only lived 3 doors up, so if there was a real problem, I was still around iyswim.
Personally, I wouldn't want to shoulder my DD with that responsibility, but everyone is different. As long as you have another policy(!) in place stating that that is what the parents want, then I don't think there's much you can do.
perhaps it does depend on individual children and what the route/area is like. I can envisage children i know and have known doing this and it being acceptable. In fact did it myself when i was 10 and i'm not that old. I'm not a cm and find it odd you'd have rules telling you that even if the parents say its fine and if it is not unreasonable parenting that you shouldn't do it. Surely they can give permission in writing.