I totally agree that we need to examine and change the treatment of children on a massive scale.
But I'm not sure that the common idea of having a damaged, unhappy "inner child" has anything to do with.. well.. children. Or even necessarily with someone's childhood. It has far more to do with how we treat adults and how adults treat themselves.
Adults are told and feel like they must not be overwhelmed by emotions, must always bear the responsibilities of surviving under capitalism, must not seek joy in unproductive activities like play, must not be vulnerable and full of raw emotion, must 'have their shit together', must have resolved all their bad experiences and memories. All of which is utterly unrealistic.
Faced with the reality of actually being are vulnerable adult who is frequently overwhelmed by emotions and responsibilities, who finds joy in play, who doesn't have all their shit together and hasn't processed everything, I think a lot of adults choose to separate that reality from their sense of self and project it onto a second, inner identity: the damaged, unhappy "inner child".
In such cases, the "inner child" can have the raw emotions that they deny themselves. The "inner child" can be flawed and vulnerable and hurt. But the "inner child" isn't actually a child, it's just an adult unshackled from the unrealistic model of maturity imposed on them.
Like, that's probably not a universal experience. Some people might have an "inner child" identity on a more fundamental level. But a lot of the time, when people talk about their "inner child" it sounds to me like the 'child' part is a fiction that separates their sense of self from their messy human reality.