To find the websites most used by your child,
Tap on the child and go to the 'Insight' page
Scroll down to Web Activities, here you can see the top three websites in terms of screen time.
To restrict usage of these websites,
Tap on the website that you want to restrict.
On the popup, only select the Routines during which you want this website to be allowed.
Don't forget to tap 'Save' before you leave!