ChildLayerSet
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.
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.