To access a node for instancing that it must be saved as a separate scene. (Right click on it in the scene tree and choose Instance Child Scene). The function
_init(): is called once a node has been initialised and cannot be used to instance other nodes. Instead you use
"path_to_the_instance_child_scene".instance(), which returns a reference to that node.
From there you can adjust any properties you like before finally adding it as a child of something (it won't be created until you do this). Here I've added it as a child of the object that is doing the instancing, but you could use
get_tree().get_root() to put it somewhere else.
var new_card = preload("res://*path of the scene to be loaded*")
export (Texture) var back_texture = "res://assets/cards/cardBack_red1.png"
var instantiated = new_card.instance()
new_card.name = self.name
new_card.mana_cost = self.mana_cost
new_card.get_child(interger_index_of_child_sprite).texture = back_texture