If you choose to complain or ask about this again, I will go to the store alone. This approach to discipline and rules empowers children. They are responsible for their own decisions. You have continued with consistency. You have concluded with a consequence.
All actions have consequences.
A grandparent cannot control the rules use by the children's parents. They can only control their own. Some grandparents are afraid of loosing the children's love and affection if they enforce rules and regulations. This never happens. Children need limits and boundaries.