The current inheritors of these classic Superman family roles may not have the same blood relationship to Superman that the former had, but they could still join the Legion just as the originals did. The 30th Century's interplanetary Legion of Super-Heroes could well invite both super-youths to join their ranks and offer them a second home in the future. The implication in Kingdom Come is that at some point they will leave our modern era behind for a more settled society of superhumans in the future. Designing these costumes was a real joy, combining the modern Legion aesthetic with the classic Silver Age outfits of Superboy and Supergirl. Superboy's current "mod" hairstyle is grown out like Superman's recent 'do, but pulled back to give a sense of the original Boy of Steel's clean-cut look.

Taken from Alex Ross's notes on Superboy and Supergirl in Kingdom Come: Revelations.