Creating a child view controller from a collection view

I am trying to create a child view for my collection view, but whenever I run the app and select a cell from the collection view, nothing appears. How do I fix this so that the tableView is then displayed once a cell is selected?

Here is the code for my collectionView.

import UIKit
class ContentViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {

let imageArray = [UIImage(named: "image 1"), UIImage(named: "image 2"), UIImage(named: "image 3")]

override func viewDidLoad() {
    super.viewDidLoad()

}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return imageArray.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CustomCell", for: indexPath) as! CustomCell
    cell.mapIconImage.image = imageArray[indexPath.row]
    return cell
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    let child = MapItemsViewController()
    addChild(child)
    view.addSubview(child.view)
    child.didMove(toParent: self)
}

class CustomCell: UICollectionViewCell {

@IBOutlet weak var mapIconImage: UIImageView!
}