My son adopted a little boy. Initially he was prepared to go along with his child seeing his biological sister, and they did see her a couple of times, but after she posted a photo of the little boy on Facebook, they declined contact, but said they could write to one another. However our grandson showed no interest in doing that, and so the relationship (which was never a big thing) died completely. Our grandson will of course have every right to search for his birth family when he’s old enough, should he wish to.
I realise it must be heartbreaking for you, but your biological grandchildren are now part of another family with grandparents, cousins etc and since you mention a disagreement with the adoptive parents, I’m not really surprised that they’ve shut down the relationship. It’s hard for you, but I think you have to accept the situation.