“Only grownups open doors”. The only problem I had was when parents were leaving for the day, they would let their kids open the door
It took just a few reminders to the parents for that one. The kids learn from the start that only adults can touch the door knob.
For a while, when they hear someone knocking, they used to yell ‘Come in’ (even though Door was locked!). Now they say ‘just a minute’