There's usually a reason that customers aren't allowed to use employee bathrooms, actually from my retail experience there are MANY reasons. A couple of examples are possibility of theft, and customers making a mess of the bathroom.

YOU are the parent, it is YOUR responsibilty to get your child to a bathroom, not Verizon's. What kind of father allows his child to urinate on the floor in a store? Actually, I should rephrase that, because I know accidents happen, so what kind of father TELLS his child to pee on the floor in a store? If I were the manager working in that store, you would be out, and never allowed to return.