How I can make the position of an object equal when you add another object?

Im doing that an empty gameObject that is empty. The localPosition.y = 0 but when I add a gameObject inside of the empty I check in the scene and is not in the same position and then I check the empty and the localPosition.y = 0 and I dont know the reason of why these happens.

Please,some one explain me why and a way to solve it. Please help me.

Did you check the Z for both objects? If they are not the same change it to whatever the parent is.