To pray the rosary in answer to a request from your grandchild was indeed the right thing to do. You must be an example to your grandchildren and show and proclaim your faith. How happy must our Blessed Mother have been to see you all gathered together in prayer. These are the memories your grandchildren will take through life and hopefully they will carry on praying the rosary and stay close to their faith. I am sorry that their mother feels that their religious upbringing belongs to her alone, for my own grandmother was very religious as is my own mother, and we learn by example. Continue to show them how you live your faith and teach them the truth no matter what barriers are put before you.
Enter by the narrow gate; for the gate is wide and the way is easy, that leads to destruction, and those who enter by it are many. For the gate is narrow and the way is hard, that leads to life, and those who find it are few. Matthew 7:13-14
May God bless you, your grandchildren and their mother