In iOS 12, Restrictions are now called "Screen Time"
Restrictions for installing and deleting apps are listed under "Prevent iTunes & App Store purchases"
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You can also prevent your child from being able to install or delete apps, make in-app purchases, and more. To prevent iTunes & App Store purchases or downloads:
- Go to Settings and tap Screen Time.
- Tap Content & Privacy Restrictions. If asked, enter your passcode.
- Tap iTunes & App Store Purchases.
- Choose a setting and set to Don't Allow.
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from: Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support