My sons have been adopted from the care system, like about 2000 children a year.
Like most modern adoptions, we have "letterbox" contact with birth families and we send photos. For people on the edge of literacy, a photo of a happy and well adjusted child seems a very little thing to give, when we have the pleasure of the chilkdrens' company for the rest of thier lives.
However, some members of the extended family do not agree with the adoption. And, sometimes, birth families have been known to seek out children and try to reclaim them.
With that in mind, how do I feel about all the parents taking photos of my child next to theirs in the school play/sports day etc? The answer is, anxious. Let us suppose that they put their photos on facebook. And suppose that a friend of a friend sees them and says, ooh look, there's our Fred: I didn't know he goes to Hogwarts. Let's stand outside Hogwarts and see if if we can find him."
How can I stop this happening? Well I can ask the school to stop all parents taking photos, and I have done this till I am blue in the face. I can say, little Jimmy cannot be Joseph in the nativity/captain of the football team -- which is pretty tough on little Jimmy because it is the only thing that he is good at.
Or I can do what I have had to do to keep my child safe, and say to his poor sad and pathetic family, no you cannot see any more photos. You will just have to get someone to read the letters I send.
Now do you all understand why schools stop parents taking photos of their children? I only wish my school would.