Stress is a physical/physiological response of the body to a difficult situation and/or stressors.
Stress is mediated by a hormone called cortisol (also called the stress hormone).
All events including lack of control, unpredictability, novelty or threatened ego are likely to cause stress in your child. This is a move, a new or disturbing experience, the changing attitude of a loved one.
So the excessive stress of parents is also a stressor for the child.
Traumatic experiences are also a source of stress in children, such as the death of a loved one, or a car accident ... The stress will be engraved in the memory context of the event.