For starters, Zoro is usually seen as a tough and reliable member of the crew. As a baby, the crew would feel a stronger sense of protectiveness over him. His relationship with Sanji could become less confrontational as Sanji would see him as a helpless baby. With Luffy, they'd have a different kind of bond. Luffy would be excited to have a 'new' crewmate in this baby form of Zoro and would want to include him in all kinds of fun activities on the ship. The female crew members like Nami and Robin would take on a more motherly role, and Zoro would be more reliant on them for basic care, which is a huge change from his independent self.
His relationship with Luffy would become more of a big - brother - little - brother dynamic. Luffy would likely play with baby Zoro a lot. Sanji might be more nurturing towards baby Zoro, which is a big shift from their usual banter. Nami and Robin would probably dote on him more, and Usopp might try to come up with little games for baby Zoro.
His relationships with his male friends might become more complicated. Some might be more protective of him as a girl, while others might be confused about their feelings towards him. For example, if he had a rival before, that dynamic could shift completely.
His relationships would change drastically. His work colleagues, for example, would notice his different behavior. He might start to work at night and avoid daylight. His relationships with other vampires could also be complex. He could form alliances with some vampires who are more sympathetic to his situation, while others might see him as an outsider. In his social circle, he would become a mystery to those who knew him before, and he would have to find new ways to interact with people.
Inuyasha's relationship with his enemies would also change. Some enemies might underestimate him in his baby form, not seeing him as a threat. However, some might see it as an opportunity to get rid of him once and for all. His friends would have to be extra vigilant in protecting him. And as Inuyasha grows up again in the story, his relationships with his friends would be different because of this experience. They would have a shared memory of taking care of him when he was at his most vulnerable, which could deepen their loyalty and friendship.
His relationship with Sasuke would likely be affected. Sasuke might initially be suspicious of Naruto's new form, thinking it could be a trick by an enemy. But as time goes on, they could find new ways to work together. Naruto's new Kitsune powers could complement Sasuke's skills. For example, Naruto could use his enhanced speed as a Kitsune to keep up with Sasuke in combat.
His relationship with MJ would become very different. Instead of a romantic partner, she might become more of a protective caretaker. She would have to deal with a baby - like Peter, which could be both challenging and endearing.
His relationship with Sakura might change. Sakura could be initially shocked by his new look and it might make her see him in a different light. Maybe she starts to notice him more as a person rather than just her friend. For example, she might become more curious about what happened to cause the change and in the process, grow closer to him.
His relationship with Ron would definitely change. Ron might be a bit more hesitant around her at first, not really knowing how to act. But as time goes on, they could develop a deeper, more platonic bond. It would be like Ron having a sister all of a sudden.
Well, with Dipper becoming a girl in the fanfiction, his relationship with Mabel would have a new dynamic. Mabel would be all about embracing Dipper's new form. She'd be planning slumber parties and trying to give Dipper a new 'girly' identity. As for Wendy, Dipper might find himself more in tune with Wendy's interests on an emotional level now that he's a girl. Wendy might be more open to sharing her feelings with Dipper - girl. His relationship with the townspeople could also change. Some might be more accepting, while others might be a bit more judgmental. And his relationship with the supernatural elements in Gravity Falls could be different too. Maybe some entities are more attracted to his new form, while others are repelled, which would lead to a whole host of new adventures and challenges in their relationships.
Allura and Lance's relationship would also shift. Allura might be more motherly towards baby Lance. She'd use her elegance and grace to take care of him, perhaps singing Altean lullabies. Coran would be the wise old caretaker, sharing his knowledge about the universe with baby Lance in a simple way. Pidge might see baby Lance as an interesting experiment at first, but would also grow to care for him deeply as she realizes he's still their friend Lance, just in a different form.
His relationship with Toothless would change drastically. Instead of being the rider, he becomes like a little sibling or a young apprentice to Toothless. Toothless would have to be more protective and patient with Hiccup.