Circus Baby is not from this game. Its from FNAF: Sister Location. The only part she appears is when the Desk Man says:
-What are you doing here? Can't you see im busy?
-You deactivated my games?
-I didn't know what else to do. I don't want to disappoint.
-Something terrible is coming...
-Her name is Baby. [<-- Thats Circus Baby]
-I am afraid there is nothing you can do. It is too late to deactivate her.
This cutscene along with the message to be continued led players to believe that there will be a third update for FNaF World however Scott made a post on gamejolt that there will be most likely no update 3, even though Update 3 may never happen a fan game exist that continiues the story from FNaF World. The game is still a demo, you can download it here.