I am wondering if two Catholic grandparents who have power of attorney signed by both biological parents can have a minor grandchild baptized.
If they are raising their grandchild with the legal consent of the child’s parents, yes, they probably could. A baptismal celebrant probably would be concerned about whether or not the situation would endure and the child’s Catholic education would be reasonably secure. The grandparents should discuss the matter with their pastor. They could also contact the St. Joseph Foundation, a Catholic apostolate specializing in issues of canon law, for assistance.