The inner child refers to a soul fragment, or multiple fragments, that split from you when you were a child.
The issue is that these fragments are emotionally and logically immature, yet they can gain considerable power. They have the same mental and emotional capacity as you did when they split. Especially young ones, such as babies, can be excessively violent or manipulative and have a virtually endless hunger. The younger the split was, the less understanding and compassionate the inner child will be, because they have not yet matured enough to develop such understanding.
The solution is love. Unconditional love. You must love your inner child as you would your own child, and be the parent that wasn't there. Slowly then, it will heal, and eventually it will transform and reintegrate back into the whole.
Fragments that were particularly young have extreme psychic powers, which they use to get their needs met. These psychic powers will remain with them as they mature and eventually become yours as they reintegrate with the whole.
The difficulty is only in identifying what is a fragment of your inner child, as opposed to another person or a parasite that has latched onto you. Hence where wisdom merges with love. You must act as you would with a real child. Give love but do not allow yourself to be manipulated. Unconditional love does not mean enabling bad behavior. To heal your inner child is to solve the problem of parenting: when to give attention, when to resist, when to teach, when to allow, when to give in. It is an art. Yet, it is attainable.