She can believe in her heart whatever she wants. As for outward signs of a religion (church attendance, baptism, or in other cases wearing a hajib, mosque attendance, or in other cases circumcision, wearing a special hat, attending temple) are up to her parents until she is independent.
In this case, dad’s ok with it, mom’s not, I say let the child split the difference and so as she wills.
That’s the way I see it.