Can I be the voice of my now 15 yo DD OP?
She has suffered being not the favourite out of a range of all the grandchildren within the family.
She saw the inequity, it hurt her. They treated her differently, it hurt her. With this and other bs from her own father and this unequal treatment, she's left now with lifelong anxiety and feelings that there's something wrong, with HER, and she doesn't measure up. It's absolutely, totally, heartbreaking.
This will be your future reality if you don't do something to protect your children you're right, as they get older, they DO notice. I bet you'll see this reflected in family photos as well - they say a picture tells a thousand stories, and who is and isn't in them and who is grouped with who etc, can be very telling.
My advice from my own experience would be to say that to them that you've noticed the differences/favouritism and you need them to know that it's hurtful for your children. Either they rectify it by doing a b c, or you'll be going NC to protect your children from being made to feel like shit by them.
For me, it was a bit difficult to deal with as this only ever went on when my DC were having contact with their dad at his house (with visits to and by the GPs) and it finally came to a point where she was a. old enough to have her voice listened to if it had gone back to court and b. she literally couldn't take any more, she was broken, and we were at the rendezvous point and she just broke down (I don't mean she just cried a bit, she fell apart) and wasn't able to go for regular contact ever after that.
The effects of this type of bs are lifelong and extremely damaging and I sincerely do hope you're able to resolve this all in a way that works for you.