Master / Detail validations

Hello
I am confussed and cant go on, maybe somebody can explain me.
I have a Master tabel (table: planningsysteem), i have a checkbox (klaar_exp). I want to close this order but...
The Header/Details (Table: header)rows have also a checkbox (Done) and have to be all closed before you can close the order in the mastertable.
How can i do this, or validate?
Made a drawing:

Many thanks,