You’ve encountered a fundamental limitation of Parental Controls - in that you need a device from which you can “manage” applied policy, both setting controls and respond to authorisation requests.
As an alternative to implementing Parental Controls, you might instead consider using the ScreenTime controls of the iPad - setting the ScreenTime passcode to something that only you know.
Use Screen Time on your iPhone, iPad or iPod touch – Apple Support
Set up Screen Time for a family member on iPad - Apple Support