// 1. A property has the same name as a segue identifier in XIB
@property (nonatomic) ChildViewController1 *childController1;
@property (nonatomic) ChildViewController2 *childController2;

// #pragma mark - UIViewController

- (void)prepareForSegue:(UIStoryboardSegue *)segue
                 sender:(id)sender
{
    [super prepareForSegue:segue sender:sender];

    // 2. All known destination controllers assigned to properties
    if ([self respondsToSelector:NSSelectorFromString(segue.identifier)]) {
        [self setValue:segue.destinationViewController forKey:segue.identifier];
    }
}

- (void)viewDidLoad {
    [super viewDidLoad];

    // 3. Controllers already available bc viewDidLoad is called after prepareForSegue
    self.childController1.view.backgroundColor = [UIColor redColor];
    self.childController2.view.backgroundColor = [UIColor blueColor];
}