We were all at one time children. And the child you once were did not just up and vanish, to be replaced by a perfectly formed, always responsible adult. Definitely, most of us act very much like children at least now and then. It's in the moment you have a temper tantrum over a parking ticket, or while driving or on working place etc. We might look like an adult, but inside is an angry five-year old who trusts no one. We forget that there is a big difference between a child and an adult. A child is loved for their tantrums while an adult hated. An adult has to think as well as behave like an adult. It is not only the age difference between a child and an adult but also that of behavior and thinking.
'Inner child work' is work with a therapist to resolve the childhood emotions and experiences this 'inner child' still holds, as well as harness the joy, innocence, and confidence that were your birthright.