Hi Sam,
When the Outline mode is displayed vertically we only show the separating add buttons for the top level elements (Bars in Headers/Footers and Sections in other builders). We tried some variations with the inline buttons at every level but it started to feel very cluttered and adding elements is more of a secondary action to organizing them.
You can add a Container by clicking the + icon found within a Bar’s outline item. That will add a Container to the end. You can also drag the workspace to the bottom of the screen and work with Bars/Containers in the horizontal version of the Outline which is more like how Headers/Footers were setup originally.
I see what you mean about not have the “Children” control like the other Layout elements in other builders. We could add one of those to the Bar as well for parity.