How do you play as Robin in Batman Arkham City?

I was told that you could play as Robin in Batman Arkham City but I can't figure out how to do it. I have him and Catwoman unlocked just don't know how to play as either oneHow do you play as Robin in Batman Arkham City?
im not sure if you can play as robin in the free roam b ut you can do it in the challange maps just go to the ridler challanges in the main menu. And to play as cat woman just go to a character switch point its like 3 cats on a room theres 2 or three around the city. Ik one if in jokers fun land and anothers by mr. freezes place hope i helps