I found this post helpful, which is simple and easy to do. https://facepage2.blogspot.in/2017/12/how-to-restrict-safari-from-launching.html?m=1

I found this post helpful, which is simple and easy to do:

1. Go to settings.
2. Search "guided access" (or go to General > Accessibilty > Guided Access)
3. Click on Guided Access
4. Turn on Guided Access

Then just go to Safari and triple click home button to enable Guided Access and surf any website without letting Safari to launch apps.