Can we have our granddaughter baptized if her parents won't?
The short answer
Not unless she is in danger of death. Canon law requires that at least one parent consent and that there be a founded hope of a Catholic upbringing; only in danger of death may an infant be baptized against the parents' will.
Canon 868 says that for an infant to be baptized lawfully, (1) the parents, or at least one of them, or whoever legitimately takes their place, must consent; and (2) there must be a founded hope that the child will be raised Catholic; if that hope is altogether lacking, baptism is to be delayed and the parents told why. An infant of Catholic or even non-Catholic parents may be lawfully baptized in danger of death, even against the parents' will.
So unless your granddaughter is in danger of death, you may not have her baptized against both parents' wishes. At five, she still counts as an infant, since she has not reached the age of reason, customarily understood as seven.