Damned if you do and damned if you don't! The reality is that ANY dog left tied up while the owner pops in to the shops has the potential to bite someone - forget about that someone having a modicum of commonsense to NOT approach a dog they don't know. Same goes for the family you encountered - stupid parents letting their child go up to your dog.
No matter how wonderful a temperament your dog has, the sad truth is that the general public judge your dog by how it looks, not by it's temperament. Sadly it means that you may have to be a little more careful about taking her out in public - god-dog-son is an AmStaff x and he is the sookiest most gentle dog, yet people are terrified of him, and his owners are terrified that the BSL might have him seized.
In the meantime give your dogs a cuddle from me to make up for the idiots out there!