A pp who said this is distributing 'child pornography' is incorrect. Child pornography is an oxymoron. Sending sexual images of children is child abuse.
The law says
"In the UK the age of consent for sexual intercourse is 16. However, it is an offence to make, distribute, possess or show any indecent images of anyone aged under 18, even if the content was created with the consent of that young person. The law is contained in section 1 Protection of Children Act 1978.
Examples include:
a child (under 18) sharing a sexual image with their peer (also under 18);
a child (under 18) sharing a sexual image created by another child with a peer or an adult;
a child (under 18) in possession of a sexual image created by a child (under 18).
So, both of the children are guilty, her by sharing a sexual image, has m by being in possession of a sexual image created by a child.
I also think the suggestion the same PP made about telling the mother that you may go to the police if she decides to take this further is shocking. It's almost akin to blackmail. Both children have done a very stupid thing,which is now coming back to bite them. Neither is innocent. Your DS didn't delete the image as soon as he received it, because if he had, your DH wouldn't have found it.