Males and females have the same appearance, except that males have a red crown which is sometimes visible. The name Kinglet derives from these tiny crowns. When fully displayed it's a large (for them) red patch on their head. They typically display the crown when they are agitated or when displaying it to one another for whatever reasons they like to show off their crowns to other Kinglets.
They are Winter birds here in Austin, so while they're here I've been trying to get a decent photo of one. So far this is my favorite. You can see a little bit of the crown on this male.