So I started looking into what makes children cut.
Totally random cute ape picture
While not all people who experienced trauma at a young age cut, many cutters did experience trauma to one extent or another. Two things that help to integrate and overcome trauma (or prevent its aftereffects) are a sense of control (ability to change outcomes) and an ability to make sense of the event(s). Infants and small children really have no way to control their situations. They can't feed themselves, nor can they change their own diapers. They also don't have the reasoning capacity to make sense of what is happening to them. They just don't have the experience or the knowledge. So the brain takes over and goes into survival mode by inducing a dissociative state which "removes" the child from feeling the pain. This state is brought about by the brain flooding itself with opioids such as endorphins and enkephalins which are natural heroin-like substances that kill pain and produce calm. They are an integral part of the brains stress-response system.
Since the brain "learns" from repetition, it learns that the only way to deal with anxiety and stress is to release these opioids (since it has no other way to deal with it). Cutting produces stress on the system through pain, which forces the brain to release these opioids. So when a child cuts, they are typically in a state of anxiety or stress that is so great they are essentially self-medicating the only way they know how.
How does this help? If you know your child is cutting, you can attack the problem by reducing their stress and anxiety for the short term. The long term solution is to re-train their brain to provide them with other, better tools to deal with the stress. Meditation, yoga, mindfulness are all wonderful tools, as is encouraging writing or expressing themselves through art (and you thought they were just making pretty pictures to hang on your fridge!!). Talk therapy can also be helpful, but keep in mind that therapists can do far more harm than good with RADs. If you can't find an Attachment therapist, and the therapist you do find isn't willing to work with you and learn about attachment issues, it may actually be better to go without for the time being (disclaimer: I'm no doctor, I can't give medical advice, and you need to do what's right for you and your child. I can only speak from experience).
However you choose to approach it, keep in mind that the cutting is a symptom - the cause is the lack of ability to effectively deal with stress. Treat the cause and you'll stop the cutting. It worked for us :)