UIView animateWithDuration: animations: completion:
    UIView transitionWithView: duration: options: animations: completion:

@property (nonatomic, strong) FirstViewController *fVC;
@property (nonatomic, strong) SecondViewController *sVC;

self.fVC = self.childViewControllers[0];
self.sVC = self.childViewControllers[1];

self.fVC.view
self.sVC.view