Double-faced cards don't have a "face-down" side; Ixidron does nothing to them.
711.6. A double-faced permanent always has the status “face up” (see rule 110.6). Double-faced permanents can’t be turned face down. If a spell or ability tries to turn a double-faced permanent face down, nothing happens.
Cloistered Youth for reference
Commander/EDH Decks:
BRG The Blood of Jund - Kresh the Bloodbraided BRG
WR The Blades of Goldnight - Gisela, Blade of Goldnight WR