App installation is completely independent between devices. Installing or deleting Apps on one device will not impact other devices in any way.
However, if both devices are signed-in to iCloud and other Apple services with the same AppleID, with data synchronised between devices, your personal and sensitive data (including stored passwords) can be accessed across all devices that share the AppleID. Data deletion can be replicated between devices!
You would be best advised to use a different AppleID for the iPad that you make available to others - such as your grandson.
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
If you choose to do so, you can still selectively share purchase Apps, subscriptions and Apple services using Family Sharing:
Family Sharing - Apple
Set up Family Sharing – Apple Support
Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support
How to create a new Apple ID – Apple Support
https://www.imore.com/how-set-up-family-sharing-iphone-ipad