I always imagine that everyone has an inner child, mentally healthy or not. I think it's healthy to acknowledge your inner child, it's where the magic lives, and the time to play and be innocent and become immersed in happy things. Being an adult can be dreadfully dull lol

There's another side to inner child, therapy wise where you can access the young part of yourself where the hurt is stored. If that's when whatever you're in therapy for started.

Personally, i don't have parts as per split-personality or DID have (i hope i've used the correct terms) my inner child is me accessing the playful side of my personality by playing on swings, painting, playing with friends children and being silly. However in therapy i am very much aware that i have certain ages i feel underdeveloped in, like i didn't fully complete some stages and that has left me feeling younger than my chronalogical age in certain situations.