Although I know this can be hard on a grandparent, the Church has always taught that the parents are the primary educators of their children. They also make vows to raise their children Catholic. This obviously includes ensuring they receive the sacraments of the Church.
To my knowledge, there is no way for you and your wife to educate your grandchildren on the Blessed Sacrament without:
- the parent's permission, and
- the Church's approval
before they could receive Holy Communion in Church at Sunday Mass. If one of you is a Godparent there is no reason why you couldn't teach them about the Eucharist, but the role of the Godparent is to support and aid the parent, not to be the parent.
Being a Godparent, myself, I find this very hard at times because I'll be disagreeing with what my brother or his wife will do, or not do, within the household. In trying to come across as the funny, humorous uncle, who happens to be religious, I hope to keep a good relationship with my niece and nephews. From my personal view, the religious freak in the family has to learn to chill out and learn what I did from Steve Wood ten years ago:
You can't transmit religious values without first establishing a good, solid relationship with your family members.
The Church's approval is critical in your situation because the Church has no way of knowing whether what you are teaching your grandchildren is correct [and/or] complete. The same would hold true for the other sacraments of the Church.
The important keys in situations like yours is to:
- not to burn the bridge between relatives
- have a good healthy relationship with your son or daughter and his/her spouse, and
- pray for them on a regular basis.
This is what the Catechism tells us:
CCC 1653 Parents are the principal and first educators of their children. In this sense the fundamental task of marriage and family is to be at the service of life.
CCC 2220 For Christians a special gratitude is due to those from whom they have received the gift of faith, the grace of Baptism, and life in the Church. These may include parents, grandparents, other members of the family, pastors, catechists, and other teachers or friends.
"I am reminded of your sincere faith, a faith that dwelt first in your grandmother Lois and your mother Eunice and now, I am sure, dwells in you." (2 Timothy 1:5)
I hope this helps,