No clue... the best thing I could guess is that, in the ancient practice, what we have "altar boys" or "altar servers" do was actually done by the slew of subdeacons. There is no reason to wear the orarion for them, although the way they wear it is different from how the subdeacon and deacon wear it... Some churches don't have their kids wearing the orarion. As far as the black riasa, it is theoretically only for the deacons, priests, and bishops, as it was the garment of clergy streetwear.. now they have seminarians and some tonsured readers wear it. But the riasa (anderi as we say in greek) is not a liturgical garment.