WTBH,
You are the one they will spend most of their time with, you will form their reality, also if they feel threatened or pressured then IMO it's ok to tell them to call you, D6 already knows that grammy has hurt her more than once, she will share that with D3 and as time goes on they will ignore more and more of dad an grandmas quacking.
Don't sweat this, you are a great mom, you will prevail!