The child could be a prophesied one. In the world of Percy Jackson, prophecies are common. So, this child could be the one foretold in some ancient prophecy that is linked to Tartarus. Percy finding the child sets off a chain of events that will either save or doom the world. Maybe the child is the key to closing a portal in Tartarus that is releasing evil into the mortal world, and Percy is the only one who can help the child fulfill this great task.
The child could be a young demigod who got lost on a quest and ended up in Tartarus. Maybe they were trying to prove themselves and got in over their head.
The plot might involve Percy having to protect the child from the various horrors of Tartarus. Maybe he has to fight off monsters while trying to find a way to get the child out safely.
One possible plot could be that Percy stumbles upon a lost or abandoned child while navigating the dangerous Tartarus. He then has to protect the child from the various threats in Tartarus, like monsters and the harsh environment. Maybe he forms an unexpected bond with the child as they struggle to survive and find a way out together.
The child could be an orphan from a far - off land that Percy visited. Maybe Percy found the child alone and starving, and felt responsible for the child's well - being, so he brought the child back.
One of the main challenges would be the dangerous environment. Tartarus is filled with all kinds of monsters and hazards. Percy would constantly have to protect the child from these threats. Also, the lack of proper resources like food and clean water could be a huge issue. There's not much in Tartarus that is suitable for a growing child.
Perhaps the child is the offspring of a Titan. This would make for an interesting story as Percy would have to deal with the potential danger and mistrust from other demigods. But Percy, being Percy, would see the good in the child and adopt them regardless. The child might have unique powers that set them apart from other demigods, like the ability to control time or space.
It could be a child of a minor god who has been overlooked. Maybe the child of Hecate, a young demigod with some really interesting magic powers that Percy and Artemis see potential in.
Maybe it could be a new character. A mysterious woman who Percy meets during one of his adventures. This would add an element of mystery to the story.
It could be a child of one of Naruto's friends who passed away. For example, if Shikamaru died in a battle, Naruto might take in his child. Naruto would then try to pass on Shikamaru's wisdom and strategic thinking to the child, along with his own ninja skills.
It could be Aphrodite. Given her domain of love, it would be an interesting take. Percy might be charmed by her beauty and grace, and they could end up having a child. The child might inherit Aphrodite's charm and Percy's courage.