The baby could be a half - demon, related to some other important character in the series. Maybe it's the offspring of a character Sesshomaru has some history with.
One great fanfiction where Kagome finds a baby Sesshomaru could be 'Kagome's Unexpected Discovery'. In this fic, Kagome stumbles upon a baby Sesshomaru in a forest glade. The story then delves into how she decides to take care of him, despite their previous enmity. It's a heartwarming tale of an unlikely bond forming between the two.
I'm not sure of a specific titled fanfiction exactly like this, but there could be many on platforms like Archive of Our Own. You can search for tags like 'Sesshomaru', 'Baby Kagome', 'Found Family' to narrow down the results. It's likely that in such a fanfiction, there would be a lot of cultural clashes as Sesshomaru is from a very different world than Kagome, which could make for some interesting plot points.
A plot could revolve around Sesshomaru's loss of his demonic powers. As a human, he has to find new ways to protect himself and those he cares about. He could start learning human fighting techniques or rely on his wits more. There could also be a plot where he discovers new aspects of the human world that he was oblivious to as a demon, like art or music, and these new interests change his view of humans and the world in general.
One common theme is family. Despite their often - antagonistic relationship in the original series, fanfictions like to explore how they could have a more positive family - like bond when Inuyasha is a baby. Another theme is redemption. Sesshomaru might be seen as redeeming himself from his past cold - hearted ways by taking care of Inuyasha.
Protection is also a big theme. Sesshomaru is a powerful demon, and in these fanfictions, he often uses his strength to protect Rin and their baby from various threats. Love is another common theme. The love between Sesshomaru and Rin is extended to their baby, and the story might show how this love shapes the baby's life.
There could also be humans who are afraid of the baby's mixed heritage. They might view it as an abomination and try to get rid of it. These humans could be part of a more superstitious group that believes demons and humans should never mix.
One possible plot could be that Sesshomaru, initially annoyed by the presence of the baby, gradually feels a sense of protectiveness. Maybe the baby has some special power or is related to an important prophecy. As he travels with the baby, he has to fend off various enemies who either want to harm the baby or use it for their own gain.
Maybe there was a magical mishap. A powerful wizard was trying to cast a spell on another dimension but it backfired and hit Supergirl instead, turning her into a baby. In the fanfiction, Kara's friends like Lois Lane could try to take care of her in a more human way since she has lost all her powers as a baby. They would have to keep her identity a secret too.
One possibility could be that a powerful spell was cast on the superheroine by a villain. Maybe the villain wanted to weaken her and thought turning her into a baby would be the best way. In the fanfiction, the superheroine's friends would then have to figure out how to reverse the spell while also protecting the now - helpless baby version of her.