If you intend to manage a child's iPad - such as using Parental Controls and authorising App purchases - the parent/adult requires an Apple device of their own with which to perform the necessary actions. It simply isn't possible for the responsible adult to manage a child's device from the child's device; alerts and associated actions (e.g., authorising an App purchases) cannot be actioned from the device making the request.
Outside of Educational and Enterprise establishments, iPad and iPhone are principally intended to be a single-user device - and are not intended to shared in the same manner as you may be accustomed when using a Windows PC or Mac computer which has multiple user profiles/accounts.
For managed environments, where multi-user usage is possible, the Deployment Guide provides appropriate guidance.
Shared iPad overview – Apple Support
For unmanaged environments, be aware that if you sign-out of iCloud and other Apple services - and sign-in with a different AppleID - you’ll not be able to change back for 90-days. So as to prevent account abuse, some critical Apple services will only allow one change of signed-in AppleID (on a device) in a 90-day period.
You may find the following links helpful:
Apple for Kids - Official Apple Support
Help your child set up an iPhone, iPad, or iPod touch - Apple Support
Family Sharing and Apple ID for your child – Apple Support
Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support