Developmental Trauma Disorder is a mental health diagnosis for children and adolescents that is not an official psychiatric diagnosis but that therapists can use to help them understand the complicated reactions that children have when they have experienced psychological traumas like abuse or violence. These trauma-related reactions include difficulty in managing emotions such as anger and anxiety, impulsive or oppositional behaviours, and difficulty with trusting other people and feeling secure in close relationships.
https://istss.org/public-resources/trauma-blog/october-2018/when-nowhere-is-safe-the-traumatic-origins-of-deve

Developmental trauma can occur at any point in childhood or adolescence. If psychological trauma occurs in the first years of life, from infancy through the preschool years, this can have an especially strong impact because those are the years of the most rapid brain development. However, psychological trauma also can have a very strong effect later in childhood and adolescence because brain development doesn’t fully gel until late adolescence or early adulthood, and those are the years that youths form their core sense of who they are (their identity) and how they are going to handle their relationships.

I use a framework that I developed using many different psychological theories, the TARGET model (Trauma Affect Regulation: Guide for Education and Therapy). TARGET helps traumatised people understand how the brain adapts to cope with trauma and provides a road map for recovery with a 7-step process that is summarized by the word FREEDOM. A book I wrote with a life coach colleague, Hijacked by Your Brain, explains the FREEDOM process. https://www.barnesandnoble.com/w/hijacked-by-your-brain-julian-ford/1110866456

I help clients who are troubled by shame and self-blame to look back at past traumatic experiences through their eyes as an adult who knows things and has abilities that they could not have known or been able to do in those past traumas. My goal is to help them not only experience compassion for themselves but also to identify ways in which they were strong, resilient, and caring in the midst of those traumas – and to understand that at the time they may have felt powerless or blameworthy but they were doing what they felt was necessary and possible just to get through those terrible experiences. I often find that the client actually did, or wanted to do, things to protect others but felt that they had failed when they couldn’t prevent or stop the trauma. In fewer cases I’ve learned of hurtful or even harmful actions that a client has done (or failed to do), and then we examine how they are (or can) do things in their current life that–while not re-writing the past—are consistent with the values that they feel they betrayed in the past.

Developmental Trauma Disorder (DTD) is different than complex PTSD (C-PTSD) because DTD includes many symptoms that are not included in C-PTSD. C-PTSD has been narrowed down to 6 symptoms of PTSD (such as avoiding reminders of past traumas and feeling on guard) and 6 symptoms that are more complex (such as feeling emotionally numb, distant from other people, and worthless). DTD includes very similar symptoms as those, but also problems with dissociation, unexplained bodily symptoms, behavioural problems with impulsivity, self-harm, and aggression, and insecurity and enmeshment in relationships. DTD also describes the symptoms in terms that are developmentally specific to childhood, rather than for adults as in C-PTSD.

RAD is a disorder that occurs when young children experience profound neglect and involves primarily severe emotional withdrawal and detachment. DTD involves a wider range of emotional and behavioural problems that are due to experiencing a combination of disrupted attachment (which may include neglect but also other problems such as emotional abuse) and traumatic victimization (which may include abuse, violence in the family or community or school, or exploitation such as trafficking).

I suggest you check out a webinar on the National Child Traumatic Stress Network’s Learning Center that was presented by Dr. Peter D’Amico, who is a wonderful clinician and expert in working with children and families with IDD and ASD (https://learn.nctsn.org/enrol/index.php?id=566). The webinar is free, you just have to create an account on the website using the link provided.

Polyvictimizatiion is a rather technical term so I would instead suggest readings about Adverse Childhood Experiences (ACEs) which describe how experiencing several types of adversity leads to stress that can result in psychological difficulties and that place a strain on physical health. https://www.aap.org/en-us/documents/ttb_aces_consequences.pdf

I provide a consistent message to them that I will take responsibility for anything I have said or done, or failed to do, that is making them feel disrespected or unsafe, and that I believe we can together work out any problems that are leading them to act in a threatening manner – but that there is no place in therapy for threats so they must be willing to work with me or we will have to stop until they are able to honor therapy as a safe place. I’ve given examples of similar challenges that I and other therapists have faced in the new book, Crises in the Psychotherapy Session (https://www.apa.org/pubs/books/crises-psychotherapy-session).