I’m sorry to have to speak strongly to you; I know that you probably meant well and are hurting over the fact that your grandchildren are not being raised Catholic. But here’s the situation that has been created:
I’m afraid that you have committed a grave injustice against the children’s parents and against the Church by baptizing the children on your own, without proportionately serious reason to do so (e.g., immediate danger of death)—the result being that your grandchildren have been placed, through no fault of their own, into an irregular canonical situation. Should your grandchildren ever desire to become Catholic, a priest or perhaps a bishop will need to review the situation and seek to regularize your grandchildren’s sacramental initiation.
I can only recommend to you that you apologize profusely to the parents of your grandchildren for interfering in their role as religious formators of their children and that you go to confession for having illicitly baptized your grandchildren. If you gain permission from the children’s parents, you can continue to take the children to Mass with you, but I hope that you will resolve to respect the parents’ wishes and cease trying to circumvent the ordinary protocols surrounding Catholic sacramental initiation.