Can a mother “assign” temporary custody to a grandparents without the fathers knowledge?

If father’s rights have been established through an Order of Paternity, then father will have superior rights to third parties, including grandparents.   If father has no order of paternity and is not married to mother, then mother has sole legal custody and can delegate that custody. Or, if grandparents actually have some sort of custody order, then that has to be dealt with. In these situations, father needs to begin with an order of paternity to establish his legal rights.