Fair point re the child not having the appropriate intention to commit any crime - no mes rea no crime.
Strict liability crimes don't require mens rea. They are committed solely by act.
The law says 'it is an offence for a person to take or permit to be taken any indecent photograph of a child; or to distribute or show such indecent photographs'. There's no consideration of age of consent or coercive behaviour, and although 12 is so young, she is still above the age of criminal responsibility. Whether the law is fit for purpose in an age of smart phones and social media is another matter, but as it stands, that's what it says.
That does not mean, as has been repeatedly pointed out, that Op's DD will be prosecuted or have any sort of legal case brought against her. It also does not detract from the fact she is absolutely a victim of a much worse crime and should be (and will be) supported through that by the police and social services. This is not her fault. But being a victim of a hideous crime isn't a legal defence that can be used to nullify the crime of taking/sharing indecent images, as unpleasant as that might be for us morally, and Op should be aware of that. Highlighting that isn't victim blaming.
Forewarned is forearmed. Op should be in possession of all of the facts so she is appropriately prepared for discussions with the police and her DD rather than it coming out later. I'd much rather be aware so that I was not blindsided if/when the police or social services mentioned it to me during what will already be a difficult conversation for all involved, or if DD found out and got upset or scared because school did an assembly or similar on this topic and mentioned it then.
No one is saying that the police are going to start calling OP's DD a criminal and say that it's her fault. They will be nothing but supportive to her as she is the victim of an awful crime. But Op as her mother should be aware so she can manage the situation and work with the relevant authorities to safeguard her child.
Op reporting is absolutely the right thing to do. You and your DD need support, and the police need to know sooner rather than later so they can track down the scum that did this to your poor little girl. You must be an amazing mother to have a relationship where she felt comfortably enough to share this with you 