Hey @Stucki_17, welcome to the world of smartphones!
In a couple of weeks I’ll be in the exact same position as your kids, my dad will also have the Fairphone as his first Smartphone. So I think I can relate (kind of?) 
Anyway, if you got any specific questions just open up a new topic!
Edit: Before opening a new topic, use the search function to see if something similar already came up. In case you got any questions in german feel free to send me a message.
I got a couple topics collected that might or might not be interesting for you to start with:
Toughts about my new fairphone
Software tools for dealing with your Android device
Ideas for fair or eco apps?
And some video tutorials on YouTube:
Getting you started with your Fairphone