In my iPhone 12 pro I do not get purchase approval notifications from my kid’s iPhone. How can it be solved?

If you miss an Ask to Buy notification, you can find the request in Notification Center on your iPhone, iPad, iPod touch, or Mac. If you still don't see your Ask to Buy requests, check which Apple ID you're signed in with, check your Family Sharing settings, and make sure that you're on a supported device.

If Ask to Buy isn't working - Apple Support