where should I get ask to buy notifications?

On all devices signed in under your or a guardian's Apple ID.

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

I'll add that recently there have been an unusual number of posts about this issue and it is possible Apple has recently changed something. I recommend providing feedback about this. Click on this link for Apple products feedback links --> Product Feedback - Apple

Apple may not respond but your comment will be read.