Well, one possible plot could be that the other characters in the Solangelo universe are super confused at first. Percy might be all like 'Whoa, a baby Nico?'. They'd have to figure out how to take care of him. Maybe Will is the most determined to find a way to turn him back and in the process, there are some really cute and funny moments like baby Nico trying to grab Will's stethoscope and put it in his mouth.
The plot could involve some kind of magic gone wrong that turned Nico into a baby. In the fanfiction, the gods might get involved. Apollo could come down and be all 'This is a strange situation' but not really know how to fix it right away. Meanwhile, the demigods are trying their best. Annabeth could be using her brains to research in the library of camp half - blood while baby Nico is just toddling around, getting into little bits of mischief like knocking over a stack of books with his tiny hands.
Perhaps in this fanfiction, there's an enemy who turned Nico into a baby as a way to weaken the Solangelo bond. Will is heartbroken seeing Nico in this state but also feels a strong sense of protectiveness. He tries different healing spells he learned from his father Apollo but they don't work at first. As time passes, they discover that it was a curse from an old, forgotten minor god. And to break the curse, they need to find a special relic that was lost centuries ago. So, they embark on a quest with baby Nico in tow, which makes the whole journey even more challenging and full of comical situations.