Sadly we had this happen to us last year.
I called the Police immediately. They sent someone from serious crime round who took the information of the account and a copy of the messages sent. They said they will see what the could find and explained that depending on the circumstances our child could also face prosecution for distributing child porn, despite it being their own nude and it's still an offence.
They tracked the IP address to an address in Canada and therefore couldn't take any further action but they passed it onto the relevant services over there and said if they were told the outcome they would share it.
We then got a phonecall two days after that saying they had reviewed the situation and wanted our child to attend the Police Station. We went and they said they would not be taking further action but gave them a talking to about what's legal and what isn't and reminded them that now their nudes are potentially on child porn sites anywhere in the world.
I informed the school, the Police spoke to the school and social services.were also involved, all requiring interviews with all of us. All services were thorough and really supportive of us and knew it wasn't anything sinister and no abuse involved.
At the beginning of this year, the Police contacted us again to advise that a further 14 children from the same school had been in the same situation with the same account and all passed over to Canadian authorities who had been in touch and they had been successful in getting the person behind it. They wasn't allowed to share any info about the person other than they were being dealt with. They can't tell us about what happened to the images either, other than they are out there somewhere.
It really taught our child lesson. We've always had parental controls, monitor usage and had a really open relationship with our child, they were able to come and tell us without fear when it happened and they realised what they had done.