Unless your granddaughter is in danger of death, canon law does not allow you to have her baptized against both her parents’ will. Code of Canon Law states:
For an infant to be baptized licitly: (1) the parents or at least one of them or the person who legitimately takes their place must consent; (2) there must be a founded hope that the infant will be brought up in the Catholic religion; if such hope is altogether lacking, the baptism is to be delayed according to the prescripts of particular law after the parents have been advised about the reason.
An infant of Catholic parents or even of non-Catholic parents is baptized licitly in danger of death even against the will of the parents. (CIC 868)
Note: Although your granddaughter is five years old, she still counts as an infant since she has not yet reached the age of reason, which is customarily understood to be seven years of age.