Not saying you're wrong at all, I have zero insight into foster care. Just trying to understand.
But why is it a problem if someone sees a family photo and notices that one kid looks different? He could just be adopted, or a friend or something. Is it something like witness protection, the children might have abusive parents that shouldn't be able to find them?
If it's a matter of not standing out, they will stand out just as much when you meet them anyway, right?