iOS adding child view controllers in Container View

ContainerView.swift

class ContainerViewController: UIViewController {
    
    private lazy var contentStackView: UIStackView = {
        let view = UIStackView()
        view.axis = .vertical
        view.spacing = 16.0
        return view
    }()
    
    override func loadView() {
        let mainView = UIView()
        mainView.addSubview(self.contentStackView)
        let views =  [SecondViewController(), ThirdViewController()]
        
        _ = views.map { child in
            child.willMove(toParent: self)
            addChild(child)
            contentStackView.addArrangedSubview(child.view)
            child.didMove(toParent: self)
        }
        self.view = mainView
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = .white
        contentStackView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            contentStackView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 16),
            contentStackView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -16),
            contentStackView.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 16),
            contentStackView.bottomAnchor.constraint(lessThanOrEqualTo: self.view.bottomAnchor, constant: -16)
        ])
    }
    
    public init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("\(#function) has not been implemented")
    }
}


class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func show() {
        let continer = ContainerViewController()
        self.present(continer, animated: true)
    }
}

class SecondViewController: UIViewController {
    var blackView: UIView = {
        let view = UIView()
        view.backgroundColor = .black
        return view
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(blackView)
        blackView.snp.makeConstraints { make in
            make.edges.leading.trailing.top.equalToSuperview()
            make.height.equalTo(100)
        }
    }
    
    public init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("\(#function) has not been implemented")
    }
}


class ThirdViewController: UIViewController {
    
    var redView: UIView = {
        let view = UIView()
        view.backgroundColor = .red
        return view
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(redView)
        redView.snp.makeConstraints { make in
            make.edges.leading.trailing.top.equalToSuperview()
            make.height.equalTo(100)
        }
    }
    
    public init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("\(#function) has not been implemented")
    }
}