Is the inner child what single trauma people have?
Therapeutically speaking, with trauma, it’s a form of avoidance. It distances what happened onto “someone or something else” / “not me, them”, which allows things too big to be looked at first person, to be looked at in the 3rd person. The goal, of course, is to eventually reduce the avoidance down to nil. Instead of it happened to her when she was three, or him when he was 25? It happened to me when I was three, or to me when I was 25. Instead of what she needs is XYZ, it’s what I need, etc.
The concept of an inner child isn’t solely owned by trauma, or PTSD/CPTSD... it’s been a metaphor, simile, or reality used by almost everyone everywhere in recorded history. As have non-anthro representations (the wolves in my heart, the tiger of my soul, the cherry blossom, the reed) all representing different aspects depending on the culture in question.
Like all therapeutic tools that make use of existing symptoms, avoidance -in many forms- can be used wisely to great effect, badly to terrible effect, or middling/mediocrely to muddy effect. Depending on the skill of the therapist themselves.