ChildParentAutoLayout.swift

import UIKit

class ViewController: UIViewController {
    var redBox: UIView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
     
        // Trying to migrate some old frame based code to Auto Layout
        redBox = UIView()
        redBox.backgroundColor = .systemRed
        redBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(redBox)
        
        NSLayoutConstraint.activate([
            redBox.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50.0),
            redBox.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50.0),
            redBox.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
            redBox.heightAnchor.constraint(equalToConstant: 50.0)
        ])
        
        let childViewController = ViewControllerB(holderViewController: self)
        childViewController.view.frame = view.bounds
        addChild(childViewController)
        view.addSubview(childViewController.view)
        childViewController.didMove(toParent: self)
        
        view.backgroundColor = .systemBackground
    }
}

class ViewControllerB: UIViewController {
    private(set) weak var holderViewController: ViewController?
    
    init(holderViewController: ViewController) {
        self.holderViewController = holderViewController
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder aDecoder: NSCoder) { fatalError("\(#file) does not implement coder.") }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .clear
        
        let greenBox = UIView()
        greenBox.backgroundColor = .systemGreen
        greenBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(greenBox)
        
        // Dispatching ahead one cycle allows the parent to be added, but feels gross
        // If I *don't* dispatch ahead, the parent isn't part of the hiearchy yet, therefore
        // has no common ancestor, and the constraint crashes.
        DispatchQueue.main.async {
            NSLayoutConstraint.activate([
                greenBox.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 50.0),
                greenBox.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -50.0),
                greenBox.bottomAnchor.constraint(equalTo: self.holderViewController!.redBox.topAnchor),
                greenBox.heightAnchor.constraint(equalToConstant: 50.0)
            ])
        }
    }
}

my ugly aproach

import UIKit 

class ViewController: UIViewController {
    var redBox: UIView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Trying to migrate some old frame based code to Auto Layout
        redBox = UIView()
        redBox.backgroundColor = .systemRed
        redBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(redBox)
        
        let childViewController = ViewControllerB(holderViewController: self)
        childViewController.view.frame = view.bounds
        addChild(childViewController)
        view.addSubview(childViewController.view)
        childViewController.didMove(toParent: self)
        
        view.backgroundColor = .systemBackground
        
        if let children = children.last?.view.subviews.first {
            NSLayoutConstraint.activate([
                redBox.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50.0),
                redBox.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50.0),
                redBox.bottomAnchor.constraint(equalTo: children.topAnchor),
                redBox.heightAnchor.constraint(equalToConstant: 50.0)
            ])
        }
    }
}

class ViewControllerB: UIViewController {
    private(set) weak var holderViewController: ViewController?
    
    init(holderViewController: ViewController) {
        self.holderViewController = holderViewController
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder aDecoder: NSCoder) { fatalError("\(#file) does not implement coder.") }
    
    let greenBox = UIView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .clear
        
        greenBox.backgroundColor = .systemGreen
        greenBox.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(greenBox)
        
    }
    
    override func didMove(toParent parent: UIViewController?) {
        super.didMove(toParent: parent)
        
        guard let parent = parent else { return }
        NSLayoutConstraint.activate([
            self.greenBox.leadingAnchor.constraint(equalTo: parent.view.leadingAnchor, constant: 50.0),
            self.greenBox.trailingAnchor.constraint(equalTo: parent.view.trailingAnchor, constant: -50.0),
            self.greenBox.bottomAnchor.constraint(equalTo: parent.view.bottomAnchor),
            self.greenBox.heightAnchor.constraint(equalToConstant: 50.0)
        ])
        parent.view.setNeedsLayout()
        parent.view.layoutIfNeeded()
    }
}

@rogelin's approach makes sense, IMO. You could set normal constraints in viewDidLoad and then the constraint which relies on the parent in didMove(toParent:):

private var greenBoxBottomConstraint: NSLayoutConstraint?

override func didMove(toParent parent: UIViewController?) {
    super.didMove(toParent: parent)
    resetGreenBoxBottomConstraint(parent: parent)
}

func resetGreenBoxBottomConstraint(parent: UIViewController?) {
    greenBoxBottomConstraint?.isActive = false

    if let redBox = (parent as? ViewController)?.redBox {
        greenBoxBottomConstraint = greenBox.bottomAnchor.constraint(equalTo: redBox.topAnchor)
    } else {
        greenBoxBottomConstraint = greenBox.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor)
    }

    greenBoxBottomConstraint?.isActive = true
}

I think it'd also be cleaner to expose a custom layout guide instead of redBox. There's an example in my fork.