Problem
- Donwnload app on childs device
- FIRST enter “Apple ID Password”.
- SECOND enter password when “Sign-In Required”
- Click “Install” button.
- “Ask Permission”
- Approve download on other device.
- Enter App Store password a THIRD time
Solution
Under Screentime > Content & Privacy Restrictions > Content Restrictions
Set to Allow All Apps.
source: https://discussions.apple.com/thread/252151928?answerId=254086237022#254086237022