Whitelist Apps in AdGuard on iPad

Sorry, under stock iOS there is no way to restrict apps except thru the parental controls you already mentioned.

Outside of the stock iOS however, there are multiple solutions to do this, my favourite of which is iUsers that enables multiple user accounts including guest accounts on iOS devices.

In a pinch, you could look through the multitasking bar that appears when you double-click the home button. The apps are arranged in order of most recently used. It is not a measure of how frequently an app is used, but it could at least tell you if it was used recently, or if other apps have been of more interest lately.