If Ask to Buy isn't working, read the suggestions in this document --> If Ask to Buy isn't working - Apple Support
From: Approve what kids buy with Ask to Buy - Apple Support
"If you miss an Ask to Buy notification, you can find the request in Messages on your iPhone, iPad, iPod touch, Apple Watch, or Mac. If you don't see the request in Messages, you can find your Ask to Buy request in Notification Center [ If Ask to Buy isn't working - Apple Support ]."
Make sure notifications haven't been disabled. This article discusses notifications --> Use notifications on your iPhone or iPad - 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 --> https://support.apple.com/HT201232