Well, it could be a really interesting story. Maybe he'd have to rely on his younger self's instincts while still having the memories of being Batman. He might struggle to convince the other Justice League members who don't recognize him as the adult Batman. And he could start to see Gotham from a whole new perspective, like a child's view of a big, scary city.
Well, Batman as a kid in fanfiction would have a very different personality. His determination would still be there, but it would be mixed with a child's innocence. He'd be more likely to make mistakes based on inexperience, but also more willing to forgive and forget. His sense of humor might emerge more, as kids tend to find humor in different things. He could also be more vulnerable, which would make his relationships with other characters more complex. For example, his relationship with Alfred would become more of a father - son dynamic, with Alfred having to be more protective and nurturing.
One idea could be that Batman is turned into a cat by a villain's new experimental device. As a cat, he has to rely on his natural feline abilities like stealth and agility to still protect Gotham. For example, he could use his cat form to get into small spaces in buildings to gather evidence or spy on criminals.
Another possibility is that Batman is experimenting with a new technology to enhance his abilities. He creates a device that is supposed to give him the ability to fly like a bat. However, something goes wrong during the testing phase. The device fuses with his DNA, and he starts to turn into a bat. He has to deal with the physical and mental changes, all while trying to keep it a secret from his allies like Alfred and Commissioner Gordon.
Well, in a fanfiction where Peter Parker turns into a kid, perhaps he could end up at his old school. He'd be faced with bullies again, but this time he has the knowledge and experience of being Spider - Man, even if he's in a kid's body. This could lead to some interesting scenarios where he uses his smarts to outwit the bullies without using his powers directly. Or, he could befriend his younger self's friends in a whole new way, seeing them from a different perspective.
Batman would be heartbroken. He has trained Robin and sees him as a partner and almost like a son. He would first try to talk to Robin, to find out what made him turn evil. He would hope that he could reason with Robin and bring him back to the good side.
Well, the consequences could be far - reaching. In the superhero world, his absence as the full - fledged Spider - Man could lead to a power vacuum. Villains might become bolder, thinking Spider - Man is gone. On a more personal level, Peter as a kid might struggle with his identity. He's used to being a responsible superhero, but now he's a child again. He might have to relearn how to control his powers as they could be affected by his new physical state. And his relationships with love interests would also be affected. They might see him as a cute kid rather than the man they were attracted to, which could create some really interesting and complex storylines.
Well, he might find it really hard to operate his Bat - gadgets as a cat. His paws are not made for pushing buttons or typing on keyboards. So he has to figure out a way to adapt. Maybe he uses his cat claws to scratch out messages for Alfred to understand what he wants to do.
Another plot could be that he is exposed to some sort of mind - controlling substance or technology. The enemy could be using it to turn him against Gotham. Under this influence, he starts to see his friends as foes. He could begin to attack the Bat - family and other superhero allies. His actions would be erratic and violent, and the city would be in chaos as they try to figure out what happened to their hero and how to stop him.
Perhaps it's due to a psychological breakdown. Batman has endured so much trauma over the years. The constant fight against crime, the loss of loved ones, and the never - ending battle with his inner demons could all take a toll. One day, he just snaps. His mind can no longer handle the pressure, and in an attempt to escape from his own tortured self, he subconsciously adopts the persona of the Joker, as the Joker seems to have no such moral and emotional burdens.