Stoick would be quite surprised and a bit worried. As a father, he'd take on the responsibility of caring for his baby son. He might be a bit clumsy at first, not used to dealing with a baby again. The other Vikings would have mixed reactions. Some might find it funny, while others would be more concerned about how to reverse the situation and get their leader back to normal.
Toothless would be extremely confused yet instinctively protective. He would curl around Hiccup to keep him safe from any potential threats. Astrid would be a mix of emotions. She would be worried about Hiccup's well - being and also frustrated that she can't communicate with him in the normal way. The other Vikings would have diverse reactions. The ones who were skeptical of Hiccup's dragon - friendly ways before might see this as a sign that his ideas were wrong all along. However, those who trusted Hiccup would try to support him in this strange new situation. They might start looking for any magical items or wise people in the village who could help reverse the transformation.
Percy would probably be very protective. He'd be constantly worried about her safety and would do everything he could to keep her out of harm's way.
He might be shocked at first. After all, finding a baby out of the blue is not an everyday occurrence.
One possible plot could be that a magic spell gone wrong in Berk turns Hiccup into a baby. Toothless, being his loyal friend, tries to protect the baby Hiccup from the chaos that ensues. The other Vikings are confused at first but then start to take care of the baby Hiccup in their own clumsy ways, leading to some comical situations.
Zazu would be flustered. He's used to reporting to the grown - up Simba and giving advice. With Simba as a baby, he would be at a loss for a while. He would try to maintain order in the pride but would also be constantly worried about Simba's safety. The hyenas, if they got wind of this, would see it as a chance to cause chaos. They might try to attack the pride while they think Simba is vulnerable. But the pride, united by their love for Simba (even in his baby form), would be ready to defend their king and their territory.
Astrid would probably be very surprised at first. She might be a bit jealous of Hiccup's new appearance as a girl, but also curious about how this change has affected Hiccup's personality. Stoick would be shocked and might be a bit overprotective. He'd have to adjust to the idea of his son being in a different form. Snotlout would likely make some inappropriate jokes at first, but then gradually come to accept Hiccup as she is. And Toothless? He might be a bit confused at first, but his bond with Hiccup would likely remain strong regardless of the physical change.
One possible plot could be that Hiccup's transformation into a baby dragon is the result of a magic spell gone wrong. He then has to figure out how to reverse it while experiencing the world from a dragon's perspective. He might make new dragon friends who help him understand his new form and search for a cure.
Hiccup would probably be very worried. He'd rush to her side immediately. He might start to check her injuries right away and call for help if needed.
The ninja in the village, like Kakashi, would need to figure out how to handle this situation. They would have to ensure Naruto's safety while also trying to find a way to reverse the transformation. They might also have to deal with any potential threats that come as a result of Naruto's new form. For example, if other villages heard about it, they could try to take advantage of the situation.
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.