結婚相 can be the photo of: 齋註冊相 or 影結婚相 or 擺酒相 or 度蜜月 or some special culture for wedding (not chinese).
The main point the school what to point out is: get marry first and then have kids is correct steps!.
Then, what can your kids do? Which photo can the kids 'stick' in the place for wedding of your parent (step 1 or 2)?
Then, the teacher will discuss the 'correct' steps for family tree in the class. Those kids may be very unhappy if they know that their parent are not marry ......... (not follow the 'standard' steps)