In regards to how to notify the other parents I'd have fired off an email immediately letting parents know what happened and confirmed at the door that they got it. There is no reason to give any indication of the child's past or her status as a ward of the state, but what you did owe the others is an explanation of what the other children saw, what was done in response, and what the plan is to prevent it from ever happening again. There is ZERO breech of confidentiality in that.
When something out of the ordinary in my day occurs this is exactly what I do. I want the parents to hear it from me, not their child. I dont want any confusion or misunderstandings. I would have asked at the door did you get my email? If they did not, I would ask them to call me or let them know that there was a very sensitive issue that occurred today and to please check the email or call if necessary.
BTW...kudos for you for being able to make a difference in these children's lives....it truly takes an amazing person to do what you are doing...