From World Wind Wiki
Icons is a special type of ChildLayerSet that includes the code necessary to render Icon objects. For an Icon to appear in World Wind it must be inside of an Icons list. An Icons list can also contain any other type of RenderableObject. All of the ChildLayerSet tags in an xml could theoretically be replaced by Icons tags and the layer would work exactly the same.
All of the attributes described for a ChildLayerSet apply to Icons as well.