how to add navigation item from VC embedded in container view

I'm coding your scenario and self.parentViewController.navigationItem does work for me. Are you sure you call addChildViewController? I just don't see how parentViewController is nil if you call addChildViewController. I have to admit though that although self.parentViewController.navigationItem works for me, figuring how who manages the navigation items, the parent or the child, is kind of tricky. With the typical pushViewController style navigation stacks each view controller tends to have its own navigation times (UINavigationItem). In my case I kind of want the parent to control some of the navigation bar items but I want to the children to add/control others.

The easiest way to do it is to create a delegate protocol in your embedded view controller that will be implemented by the parent view controller.

Create a property on your child view controller of protocol type, when you create the child view controller (i suppose you are creating it from your parent view controller) set the parent view controller as the delegate.

In the parent view controller, in the delegate method(s) add the code for adding the button.

When you want to add the button just call from your child view controller the delegate method(s) and your parent view controller will handle the button that you want to add.

If you are not familiar with delegate pattern or with protocols, there are a lots of tutorials on the internet or just check one of my previous answers that are presenting a simple way of implementing delegate here or here

In viewDidAppear

-(void)viewDidAppear:(BOOL)animated
{
    UIBarButtonItem *rightItem = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"add_baby"]
                                                                  style:UIBarButtonItemStyleBordered
                                                                 target:self
                                                                 action:@selector(addNewBaby)];
    [self.parentViewController.navigationItem setRightBarButtonItem:rightItem];
}