Well, in this new scenario, their relationship would start off on a completely different note. In his past life, they had that instant chemistry and bond based on their shared experiences. But as a baby, Percy has no such experiences yet. Annabeth would be the one with all the knowledge and memories of their past relationship (sort of). She would have to wait for him to grow up and re - discover his own identity. This waiting period could create a new kind of tension and anticipation in their relationship.
If Percy is reborn as a baby, Annabeth's role in his life would change drastically. In his original life, they were equals in many ways, fighting side by side. Now, she would be in a more dominant position at first. She could be involved in his upbringing, teaching him things about the demigod world. Their relationship would be a slow - burn. As he grows and starts to show his old self, the dynamic would shift again, and they would have to re - define their relationship, which could be full of both confusion and newfound excitement.
It would be really interesting. Since he's reborn as a baby, he and Annabeth wouldn't have the same initial connection. Annabeth might first see him as just a cute baby, not the hero she knew. Their relationship would have to build from scratch. She might be more protective of him at first, like an older sister figure, and as he grows up, their relationship could develop into a romantic one in a much more gradual way compared to their previous life.