Add object `child` to the first map and store it for the second.

def add_child(self, child, name=None, index=None):
        """Add object `child` to the first map and store it for the second."""
        self.m1.add_child(child, name, index)
        if index is None:
            index = len(self.m2._children)
        self.children_for_m2.append((child, name, index))