Inner child work, for me, began when I thought of my various memories and remembered how I felt, what happened, what I thought (at that time, as a child). We are all our experiences and thoughts and feelings throughout our lives; we didn't just suddenly become grown up when we passed a certain birthday?
My mother died when I was 3, was sick all my life, in fact, was already dying when I was in the womb, was having difficulties when my brother was an infant, two years before I was born. My first memory is of being lost when I was around two years old, and my father "rescuing" me. It's a very detailed memory, I can remember what I thought and felt and how I tried to "reason" my way to getting back home. The problem was I was two and didn't yet have enough reasoning power!
In a week or two I will be 61 and now I have enough reasoning power

I can see/trace the problems of the 2 year old me and "fix" them to a certain extent; the two year old was "trapped" in being two and had to be rescued by an adult. Not all problems we have as children have rescuers though? Too, my father found me and took me home but that doesn't do anything for my sense of my abilities and the "trapped" feeling. When I'm on my own, I can tend to doubt my abilities, even at 61.
The work I do with my "inner child", my self as I was when I was two, is to show her the future path she takes; I'm amazed at my thinking as a two year old, smirk at some textbooks that try to figure out what children who cannot talk/express themselves well yet are thinking, what they know. I got lost because I went around a bend in the creek and couldn't find a bridge across. I know now that I was not able, as a two year old, to understand about bends in the creek. The frustration I can still feel, if I immerse myself in my two year old memories, of knowing there was a bridge and knowing what direction it was down the creek and knowing it logically wasn't/couldn't have been moved and yet knowing too, it wasn't "there" and I was helpless to resolve that discrepancy is very real, actually happened to me and can do with "fixing". It lessened a great deal the day I discussed the memory with my father and he explained about the bend in the creek, that he had found me several yards down from our house; we (my brothers and I) had walked at an angle I had been unaware of once across the bridge. By that discussion, I knew my psychology and of Piaget's work and that I was too young to understand. I could convey that to my younger self and become more "whole", connect with her and assure her that she was right! She was right to feel frustrated and anxious at not finding the bridge and it
was not her fault she could not. My adult self was able to confirm my child self's feelings.
My child self memories had/have lots of incidents like that that I, as a child, did not understand. Why was I so anxious when left with my girl cousins my age who were so nice; nothing bad happened? Well, going back there with both my child memory self and my adult self, they were my stepmother's nieces and I had only known my step mother less than six months. I had spent a couple years with my father and grandmother (his mother) during my mother's illness and death and afterwards. I didn't know my stepmother very well; was she a good rescuer? Would she come back for me at their house? Who were they/their mother? I was left with virtual "strangers", despite their being called my cousins/aunt.
When I understood that, it made a lot of my behavior after that clear! As a teenager, I hated babysitting because I was alone in a stranger's house. Even today I have some trouble with babysitting my husband's grandchildren! My husband and I go together to babysit and when he goes outside to smoke, I have a bit of anxiety, being in someone else's house, alone and in charge. From work in therapy and with my younger child memories, I feel better about myself and my ability to take care of myself and my adult knowledge that a lot of my fears and anxieties about being alone don't have bad results.
I saw my therapist for two periods of about 8-10 years, with ten years between the periods. The second period I went to see her because my stepmother was getting senile and unable to care for herself (and died in 2001; I saw my therapist for the period 1996-2005). The anxiety level really ratcheted up, if she couldn't take care of herself, who would take care of "Me"? A very inner child problem/question for me and it was mostly inner child work I did for the next nine years to resolve that question.
I think of the inner child as being part of my unconscious, clothed in actual memories. You know how you dream about things that have elements that happened when you were younger? People and places appear that you recognize but were from many years ago? Dreams are unconscious and I believe many of our memories are there in our unconscious, no longer remembered. I know there were lots of "recovered" memories in my work that last 9 years of therapy. A lot of understanding about what I did remember too that had been confusing to me before.
I told my T about my memories from age 3, February 1954 (when we moved from California to Maryland through about December 1955 (my father and stepmother were married in November 1955, my mother died in July 1954)). My T's perception of what I told her? I had a hard time keeping the summers of 1954 and 1955 straight! It was a jumble to me and it came out as a jumble! My T noted that, that it sounded as confusing as it must have felt. That's where inner child work helps. As actual facts and better memories are regained, the jumble gets straightened out and "smoother".
I do not have any direct memories of my mother. I have the 2 year old memory of being lost and my father finding me (my mother was in the hospital) and I have memories of Maryland after we moved (my mother did not move with us, was air-lifted directly to Bethesda Naval Hospital where she died less than a month later) and then I have memories with my stepmother; my memories were "choppy", I had three different lives/timeline, not one. Working with my inner child and getting facts from corresponding with my father's sister, my aunt about what I didn't "know", eventually I got better connected to myself, got a single, smooth, timeline. I'm more "whole" now.