My son accidentally deleted my phone app, how do I redownload the app?

Unless I'm mistaken the Phone app on iOS cannot be deleted unless your device is jailbroken.

Use Spotlight search to try to find the app.

https://support.apple.com/en-us/HT201285

The app may be inside a folder, so look inside any folder on the home screens.

If you are referring to another app like Whats App or any third party app just sign in to the App store & redownload the app. You should be able to see past purchases in the App store app to find it, check the top of the Updates listing.

Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support

Restore a built-in app that you removed

1. On your iOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

If you have an Apple Watch, restoring an app to your iPhone also restores that app to your Apple Watch.