When using a nav and tab bar controller, why does didMove only fire once?

Question

This is the setup I'm trying to work with:

As you can see, the structure goes like this:

Navigation Controller > View Controller > Tab Bar Controller > View Controller

I'm trying to pass data through the back button and typically, all I have to do is fire didMove from the view controller with the back button like this:

override func didMove(toParentViewController parent: UIViewController?) {
    if (!(parent?.isEqual(self.parent) ?? false)) {
        print("Parent view loaded")
    }
}

However, when I place this inside TabItem01ViewController.swift or TabItem02ViewController.swift, nothing seems to be fired. If I place the code in TabViewController.swift, however, didMove is fired. I can't seem to figure out why it's getting fired from TabViewController.swift but not either of the tabs views.

---

So my question is: how do I pass data from either of the tabs to the ViewController.swift file when the back button is pressed?