THAT WAS NOT THE WISE THING TO DO.
If you send pictures without permission, YOU CAN BE CHARGED FOR CHILD *advertiser censored* DISTRIBUTION.
I don't wish to break semantics here, but the law is very specific. I realize you are trying to be very careful here, but the laws are (justifiably, in my opinion) at a point where you MUST make sure you don't cavalierly send pictures or keep such images on your computer. Accessing the page may be enough to put the images into your cache, which in many jurisdictions is just as criminal as actually having them in an intentionally downloaded file.
You may have done the family of the 12 year old a favor, but in the future, please consider the consequences from the decisions you make with regard to potentially explosive material.
WHAT I WOULD DO:
1. I would record the exact link to the photos. If the link is "dynamic" and requires a specific login sequence, make note of that. Keep that for reference.
2. Call the parent of the child. Do NOT send the photos in general.
3. First choice: Make sure that you ARE talking to the parent. Give him/her the link to the account. If the parent does not have the credentials to access the site, then the parent is committing a MAJOR mistake in parenting his/her children.
4. If the parent DOES have access and goes to the site, then have her daughter IMMEDIATELY DELETE her photo.
5. PROBLEM: You do NOT KNOW WHO ELSE has seen this photo. To that end, get a list of EVERY FRIEND your daughter has on Facebook. Every one of them may have seen the photo, and every one of them has the capability of copying the image for their own use.
6. Pragmatics: The situaion ***may*** blow over once steps 1-4 are taken above. But if it doesn't, this can be very difficult to reverse. But it MUST be tried.
Under NO CIRCUMSTANCES will you go to the police without your lawyer's advice. While in most cases the police will handle such actions with reasonable understanding, SOME jurisdictions may have NO CHOICE but to follow through with a complete, probing investigation.
I'm sorry, I can't give more advice. BOTTOM LINE: Anytime you go on cam or show an image, automatically assume that picture WILL be retained FOREVER and disseminated ANYWHERE at ANY TIME for ANY PURPOSE. This is something I can NOT STRESS ENOUGH.