To delete an object and its children, you must branch-select the object first. If you simply node-select an object with children and press Delete, it gets replaced by a null with the same name to preserve the hierarchy.
To delete an object and keep its children, you must first cut its children (see Deleting Objects in a Hierarchy) or make them the children of a different object.
Branch-select the root of the subtree you want to delete, for example, by pressing the space bar and middle-clicking on it. For more information, see Selection and Hierarchies.
