Hi there frederik295,
We certainly want to help get your Screen Time working as expected across both of your daughter's devices.
From your post, we see she has iPadOS 14 installed, but the latest is iPadOS 15.2. You can verify this within Settings > General > About. Here's a link that'll show how to first back up, then Update your iPhone, iPad, or iPod touch. If she has a compatible iPhone model, the latest is iOS 15.2. This same link also shows how to update your iPhone if needed.
If you go to Settings > Screen Time, then scroll to the bottom, is 'Share Across Devices' enabled?
Does it show Screen Time is enabled on both devices? Here's the link to help you check just in case: Use Screen Time on your iPhone, iPad, or iPod touch
Cheers!