I don’t necessarily mean whether the children are on first-name terms with their teachers, because as far as I am aware they are not. They are referred to as “Herr” or “Frau” with their surnames.
But, do they use the informal Du or the formal Sie form to talk to them?
Normal German protocol would require them to use Sie when talking to someone that they are not on first-name times with, and especially someone who is considerably older then they are.
I’m just not sure how many 6-year-olds have mastered the formal verb conjunctions. Perhaps it just comes naturally to them?