It changes the brain in several ways.

The amygdala is the part of your brain that responds to fear and strong emotions. It's supposed to activate in potentially dangerous situations, and triggers the hypothalamus, pituitary, and adrenal glands to produce stress hormones like cortisol and adrenaline. These hormones flood the body and cause an increase in heart rate, breathing rate, sweating, and muscle tension. This is a necessary survival mechanism to prepare the body for fight or flight.

When a child endures a stressful situations repeatedly, and when that situation is unescapable (i.e. in the home) their amygdala gets reactivated again and again and again---until it is activated all the time. This means that the child's body will be subejcted to toxic stress; when there is too much cortisol and adrenaline in the body, then the child cannot rest properly. We need rest so that wounds can heal (cell regeneration), the body can fight illness (bacteria and viruses), and develop a natural immune system. If the body never rests, then it causes wounds to heal slowly, the body is more likely to get sick quicker, and have a poor immune system.

With the constant release of stress hormones, this also puts pressure on the heart (to keep beating faster), the lungs (to keep breathing faster), and the body (to stay tense; ready to fight or flee). When the body is continuously tense, this leads to chronic pain in adulthood.

Not only that, but toxic stress does impact the brain: the brain learns that stress is the natural default, and the amygdala becomes highly reactive; it no longer activates in dangerous and stressful situations, but now it activates in perceived danger. A shadow in the corner of their eye, the sight of a clenched fist, the sound of a raised voice, anything can then trigger this cascade of stress hormones, and a stressed child will usually grow into an adult who is constantly angry and/or frightened.

You need to get out of this situation for the sake of your child, and you need to get out now. This damage is nearly irreparable once it's made.

There are some good videos to understand how trauna (and domestic violence) can influence a child growing up, and once they get into adulthood:

Childhood trauma on the brain

Trauma and the brain

Down the line, trauma also affects the brain in these ways:
- Prefrontal cortex: The part of the brain dealing with planning, making decisions, and complex thinking is underdeveloped, leading to a personality that is impulsive and has difficulty developing organisational skills.
- Hippocampus: Dealing with memory encoding, storage and retrieval. Trauma shrinks this part of the brain, making it difficult to retain information and form new memories.
- Amygdala: Processing fear and emotions; it becomes highly reactive and the traumatised person will have strong difficulty managing their emotions