I have two kids in my apple family. One of them used “Ask to buy” to get an app, I approved and they could install. Now when the second kid does the same I only get a “view in app store” link that takes me to the app store page of the app to install it on my phone but I can’t authorize my other kid’s request.
If you download content from a family member's past purchases, you won't see an Ask to Buy prompt. Also, if you redeem a product code, you won't see Ask to Buy prompts.
Try following these article which maybe helpful: If Ask to Buy isn't working - Apple Support
Select feedback type as Bug Report and submit here: Feedback - iPhone - Apple
I'd also suggest you to connect with the support: https://getsupport.apple.com/
This seems to be a bug, is there anyway to report it?. For the record i could approve the purchase for the second child in my ipad.
Ask to buy lacks Get button
