Once this item has been unlocked, if the current character's Tainted version was already unlocked or the current character is a Tainted character, it is guaranteed to spawn in the hidden closet in Home.
This item will appear even if Inner Child had been obtained earlier in the run, allowing Isaac to have two copies of the item.
Despite being hardcoded not to appear anywhere else for him, this specific occurrence of Inner Child will still spawn for Tainted Lost.
If this item is obtained in Home, a viable strategy is to intentionally die towards the end of the fight with Dogma. Isaac will be respawned with the size down and Speed up, and these effects will carry over to the Beast fight along with the healing provided by Dogma, making Isaac much harder to hit for the fight.
The size down from being revived by Inner Child is the same as if Isaac had collected Pluto.
Inner Child is a concept in popular psychology that there exists an "inner child" in every sub-conscious that contains memories of pain and trauma in youth.
Specifically, Inner Child spawning in the Home closet may be a reference to the trauma seen in the Mother ending – where Mom throws Isaac into a closet.
In the reverse, perhaps the ending is also allegorical rather than historical: Mom's constant abuse "threw" Isaac's inner child into a closet.