Okay kiddo, so let's say you have a big playground, and different parts of the playground have different rules. Maybe one part is only for kids who like to play sports, and another part is only for kids who like to draw. Each of these parts could be called a "bailiwick."
In real life, a bailiwick is kind of like that – but instead of a playground, it's an area where a certain authority has power. For example, a police officer might have a bailiwick – that's the area they're responsible for keeping safe. Or a judge might have a bailiwick – that's the type of cases they specialize in.
So when someone says "that's not my bailiwick," they mean they’re not responsible for that. It's like saying "that's not my job" or "I'm not in charge of that."
Does that help, champ?