How Can I Protect Apps from Being Accidentally Deleted?

To prevent accidental deletion of apps, go to the Settings app, select “General”, scroll down, tap “Restrictions”, tap “Enable Restrictions”, create a four digit passcode and enter it again when prompted, then toggle “Deleting Apps” to OFF.

If a paid app does get deleted, you can always open the “App Store” app, select to the “Purchases” tab, and download the app again at no additional charge.
This article provides additional details for protecting apps from being accidentally deleted.