Hey there ankitfrombaps,
Thanks for reaching out to the Apple Support Communities.
If having issues with not receiving Ask to Buy notifications on your iPhone when your son makes a purchase, check out the steps listed in the following resource that can help: If Ask to Buy isn't working
Things to try first
1. Make sure that both you and your child have updated to the latest iOS or iPadOS on your iPhone or iPad, and the latest macOS on your Mac.
2. If you turned on Focus on your device, either turn it off so that you can receive notifications, or check your Focus settings to make sure that you can receive Time Sensitive notifications like Ask to Buy requests.
3. Try changing the name of your device, and check that you and your child's devices have different names. To see and change the name of a device, go to Settings > General > About > Name.
If your Ask to Buy requests are missing
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 your Family Sharing settings, check which Apple ID you're signed in with, and make sure that you're on a supported device.
Open Notification Center on your iPhone, iPad, or iPod touch
* From the Lock Screen, swipe up from the middle of the screen.
* From any other screen, swipe down from the center of the top of your screen.
Open Notification Center on your Mac
Click the date in the upper right corner of your screen or swipe left with two fingers from the right edge of your trackpad. Notifications show up at the top of Notification Center.
Keep us updated on the results after going through the steps above, and we'll continue from there.
Take care!