You are always best to "roll with the flow " . Your relationship with these little people, depends entirely upon your relationship with their parents , so why create problems ?
No doubt when the child gets older and socialises with the other cousins, they will pick up on the name which the others call you. Even if that doesn't happen, surely it's better to keep a good relationship with the parents , and just accept the name you don't like.
Humza Yusaf's resignation speech
Air fryer - combi microwave - oven? Help!