Half-angels are part of Hebrew lore. Look up Nephilim, though God has in theory forbidden their creation, and, as portrayed, I very much doubt they'd ever ascend to true angel-hood. Still, as a Scion concept, it's pretty darn neat.
I think angelic scions would be very rare. Angels marrying (or even just mating with) humans may tread on free will. Yeah, a person has the choice (presumably) to reject the advances of an angel, but can we expect angels to be anything less than perfect?
Heh, the human mom/dad could have the aspect Touched by an Angel And a scion of an angel, the Nephilim, I think would be one of the few and far between examples of a child born out of love, out at least consensual sex, rather than force in the case of most Changelings or demonic scions.