How to access parental controls and restrictions on iPhone and iPad with iOS 12
Parental controls have changed in iOS 12.
In iOS 11 the feature was called Restrictions and was in the Settings app under General > Restrictions as seen on this video.
In iOS 12 it has changed and grouped with a new feature called Screen Time. This article shows a quick summary of the new Screen Time feature and how parental controls can be configured there.
For more detailed instructions check out the Apple Support article.