Pinning the browser
It is possible to PIN the browser on both platforms. So leaving the app to the home screen for instance is only possible after the PIN code is punched in.
iPad - Guided Access
The browser will be pinned with the help of the Guided Access feature. Users can leave the browser only after the passcode is punched in. Video manual below.
How to use Guided Access on iPad | Apple Support
Android - Pin App
The browser will be pinned with the help of the Pin App feature. The video manual below shows how to do that with the Samsung launcher (One UI). Can differ a bit for different launchers or native Android.
Samsung S6 Lite Pin an App and Lock It to the Screen - Samsung Tips and Tricks