Are you talking about in the Editor? addchild() only adds a child after a scene is loaded, and this can vary from when or what function you use to add a child.
If you put the addchild() function in the ready function, it will add whatever child you have decided to its list of children once the node is "ready" (or, basically, pretty much right after the scene is activated)
So basically, add_child() only adds the child at a designated time when you are running your project.