The antagonists in the original story might see this as an opportunity. If it's a demon antagonist, they might try to influence baby Ciel in a bad way, to turn him into a more malevolent demon. The human antagonists might be more confused. They don't really understand demons, and now Ciel is a baby demon, which is even more of a mystery to them. They might try to use this situation to their advantage in some way, but not really know how to deal with Ciel directly.
Ciel's friends or allies in the human world would have mixed reactions. Some might be horrified at first, seeing their friend turn into a demon, even if he's a baby. But as time goes on, they might start to accept it and still try to be friends with him. They would also be curious about his new demonic abilities and how he's different from before. And Ciel, as a baby demon, would be more reliant on them in some ways, which could change the dynamics of their relationships.
The plot could center around the mystery of how Ciel turned into a baby demon. There could be a search for a way to reverse it. His allies, like the Phantomhive servants, would be in a tizzy trying to keep him safe and figure out what happened. Ciel himself, despite being a baby, might still have some of his old memories and try to communicate in his own baby - like way, which could be really cute yet frustrating for those around him.
He could interact with Harry Potter, Ron Weasley and Hermione Granger. They might be curious about Ciel and his different upbringing.
Ciel would likely become more protective. He has always been in a dangerous world, and now with a baby on the way, he'll do whatever it takes to safeguard his family. Lizzy, she might become more nurturing. She was already a strong character, but motherhood could bring out a softer, more patient side of her.
Ciel would likely become more nurturing. He's used to being in a position of power and having to be tough, but with a baby, he'd soften up. Sebastian would have to suppress his more demonic tendencies. He couldn't be as cold - hearted as he usually is. His focus would shift from just serving Ciel to also protecting the new life.
Bella would be extremely loving and nurturing. She would spend a great deal of time just holding the baby, looking into its eyes, and talking softly to it. Jasper, on the other hand, would be very protective. He would use his ability to sense and manipulate emotions to keep the baby calm and happy.
Allison would use her power to create a positive environment for the baby. She could say things like 'I heard the baby is always happy' and use her power to make it so. The characters would also have to learn to work together to take care of the baby. There would be arguments about the best way to raise the baby, like whether it should be trained to use its powers early or just be allowed to grow up like a normal child. But in the end, they would all come together to protect and love the baby because they understand the importance of family, which is a big theme in the Umbrella Academy.
The friends of Natsu and Erza would also interact with the baby. Gray might make little ice sculptures for the baby to play with. Lucy could read stories to the baby. They all would be part of the baby's life, helping Natsu and Erza raise it. And the baby would respond with lots of giggles and smiles to all their efforts.
Jake, Finn's best friend, would probably be a very protective and fun 'uncle' to the baby. He might use his shape - shifting abilities to create interesting playthings or protect the baby from harm.
In this fanfiction, aside from Gohan, there could be other characters. For instance, Piccolo. Piccolo might sense Goku's power and be both intrigued and a bit wary. He could approach baby Goku to either assess the threat or try to understand his power source. Also, Chi - Chi might be involved. Even as a child herself, she could be drawn to Goku because of his unique presence and power, which could start their early relationship in a different way compared to the original story.
Obviously, Hagrid could be one. He would be so amazed and probably take baby Harry on little adventures around the Hogwarts grounds, listening intently to what Harry has to say.