Cool discussion! I found the information interesting and fun.
Ok. After some discussion, the idea of an Eldar being born with a Spirit Stone doesn't seem possible, feasible, or worthwhile for the Eldar. Though through discussion, an interesting scenario was given to me. BUT, I came up with an idea to expand on that and was curious what others thought.
They told me that Daemons are always eager to possess a living host to use as a vessel in the real world, and Eldar in particular are favorite victims of Slaaneshi creatures. If the would-be host has the mental fortitude to throw off the will of a daemon however, and free themselves from possession; it renders them immune to becoming repossessed. (Something the Inquisition makes a point of using if their agents find an individual capable of it.) In addition, the individual gains stronger resistances to general Chaos influence.
That's also when they said that it would be near impossible for a young child to commit such a feat, but it might be within the realm of possibility in a situation such as: A pair of Eldar (the other being a friend/family/mentor/etc.) are sheltering themselves from a Chaos assault on their Craftworld, an assault undertaken by an ambitious champion of the Dark Gods. Warp rifts are opening all over, pouring unholy energies into the world around them. Despite their best efforts, the pair is found by an enemy soldier.
The other Eldar and the warrior fight. The warrior is killed, but manages to damage/destroy the other Eldar's Soul Stone before falling. Without the protection offered by his/her Stone, the Eldar is exposed to the raw power of the Warp being unloaded by the rifts, crippling his/her soul and allowing a daemon of Slaanesh to enter his/her mind. A mental battle ensues, the host is overpowered, but puts up enough of a fight to damage both: their own body and the daemon's essence.
With the initial target falling apart and unusable, the weakened daemon turns on the other Eldar, seeking to consume his soul and use him as a replacement vessel. A fierce battle takes place inside his head as he struggles to hold onto both his life and his sanity as the daemon wreaks havoc. With supreme effort of will, he manages to tear the creature from his soul and cast it back into the Warp through sheer force of will (possibly sparked by the dire situation of the other Eldar / or death of the other Eldar / or supplemented by latent psychic potential / open to suggestions).
Scene ends with him being found beside the body of the fallen Eldar in the ruins of wherever they'd been taking shelter, found after the Chaos army has been routed.
Now... for the story, I am using one of my oldest RPG characters. In the recent past, I came up with siblings for him. An older sister. About 11-12 years older. Younger siblings: twins. I never came up with anything for his parents though and was working on that. My thoughts are that the mother would be an Autarch and the father would be a Spiritseer.
In this scenario, what would you think could happen of an Autarch (the mother) experienced such a trial alongside... say her own father, twin sister, or mother? The mother becomes immune to Possession and greater resistance to Chaos Influence. Is there any chance that this could be passed on to the children?