From World Wind Wiki
A ChildLayerSet is essentially a sub-branch in the layer manager. A ChildLayerSet element can contain any of the RenderableObjects, including other ChildLayerSets. A number of attributes control the behavior of a ChildLayerSet. If a ChildLayerSet inside of a particular xml already exists, then its contents will be added to the existing ChildLayerSet in the World Wind layer manager.
Note: Icons is a special type of ChildLayerSet used to hold Icon objects. An Icon object inside of a normal ChildLayerSet will not show up in World Wind.
 ChildLayerSet attributes