Whitecloaks redirects to Children of the Light, since that is their "correct" name. So if Whitecloaks doesn't occur in this chapter, this should be just:
Children of the Light -- by the disparaging term Whitecloaks (or whatever that term should be characterized as)
However, Whitecloaks is also by far the most commonly used name. This definitely seems like a gray area.
So you could go with the characterization "common disparaging term" or something of that nature (note: I'm not insisting nor even recommending "disparaging", it's just the first thing that occured to me). In any event, assuming our article goes to Children of the Light as planned so far (by nae'blis), a usage explanation of "Whitecloaks" should figure very prominently. See help:Style Guide if you have any general suggestions about how to resolve similar gray areas.