Thank you @mingoslkd for your helpful advice.
I might have been unclear in the way I was describing my problem, which can be caused by English not being my native language, so I apologize and try again.
I had established a usage limit of one hour per day for all apps, and I had added Camera and Photos to the "Always Allowed" - list. This did grant the desired outcome of social media apps becoming unavailable after one hour of consumption whilst Camera and Photos remained available, but it did also mean that Camera and Photos remained available even after Downtime kicked in. Plus: The time my daughter spent using Camera and Photos counted against the one hour limit, thus reducing her time to use Social Media by the amount she spent looking at Photos.
So: How could I make sure that she really had one full hour to chat with her friends, could continuously use Camera and Photos - and once Downtime kicked in make sure that neither of those apps was available?
I researched the topic a little bit more, and this is the solution I found for my issues:
The error was to set the 1 hour limitation for "All Apps". This does exactly what it means. Any app that is being used counts against the 1 hour limit, irrespective if an app is on the "Always Allowed" list. Being on the "Always Allowed" list does not mean that the time spent with such an app is not counted. It only means - as @mingoslkd stated - that this app remains available even if all time restrictions are used up, and even if Downtime kicked in.
I found I had to be more specific in the limits I set: I edited the 1 hour rule so it would not apply to "All Apps" any longer, but to the group of "Social Media" apps (plus I added Netflix and Youtube). This immediately fixed my problem, and I could remove all apps from "Always Allowed" but for the phone app, which cannot be removed.
Now I have what I wanted: A fixed nightly Downtime during which the Smartphone is useless, one hour per day granted for my daughter to chat and entertain herself and the liberty to use Camera and Photos anytime during the day irrespective of time limits.
