Passcode
Is there a way to add a passcode that needs to be used when my grandson tries to subscribe to something on an iPhone or iPad?
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Presumably your grandson has his own devices - as, outside of educational environments, iOS/iPadOS devices are not intended to be shared.
If your grandson does have his own devices, you might be best advised to consider using Family Sharing. In so doing you will introduce appropriate controls on both purchase and (for minors) access and use of some features and content. You might need to consider the “family” group in a wider context.
You may find this information to be helpful with setting-up an AppleID for a minor (if necessary) - and considering appropriate controls:
https://www.imore.com/how-set-up-family-sharing-iphone-ipad
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
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
Parental Controls
Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support
Use Screen Time on your iPhone, iPad or iPod touch – Apple Support
Set up Screen Time for a family member on iPad - Apple Support