If Ask to Buy isn't working, read the suggestions in the document: If Ask to Buy isn't working - Apple Support about looking for the request in Messages or Notification Center.
Make sure notifications haven't been disabled. This article discusses notifications: Use notifications on your iPhone or iPad - Apple Support
and this one Change notification settings on iPhone - Apple Support
Make sure the software is up to date on all devices involved.
Sometimes it helps to have the devices involved take a fresh look at settings:
● Try signing out of accounts and back in again.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
Sign in with your Apple ID - Apple Support
● Restarting is a simple step that can resolve many unexpected behaviors.
iPhone: Restart your iPhone - Apple Support then choose your model.
iPad: Restart your iPad - Apple Support
Mac: Ways to start up your Mac - Apple Support
If a problem persists, particularly over several days, try contacting Apple for assistance:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
"See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support
In the meantime, if a parent or guardian is nearby, you can tap "approve it in person" and they can approve the purchase directly from your device.
