Using the main menu, "Menu: Tree/Expand" you can expand a subtree.
There are two options available inside this menu item:
"Menu: Tree/Expand/Subtree (one level)"
This will result in all children (of the currently selected node) becoming visible.
"Menu: Tree/Expand/Subtree (all levels)".
This will fully expand a subtree. Not only its children but also all its descendants will become visible.