Let's say the majority of real life kids are brats. I personally don't like real life brats. I don't like bratty main characters. But, you're creating a story. A main character is usually different from the majority. So with that formula, a main character is usually not a brat. They got some dark past going on, are getting bullied, or something else that's sad.
There could be many reasons for an "unrealistically" mature kid. But, I honestly think it's just our modern society that has made us this way. If we rewind our timeline, we would be all very different people because everything has changed. Struggles bring responsibilities. A kid growing up getting everything they want by default doesn't learn as much responsibility and may go the bratty route. I'm not sure if I should put this comparison here because I barely watched it, but look at the difference between Naruto and Boruto. Both were brats as kids, but one was brattier xD.
But, no matter what I just said. A character going through a dark past or cruel present can still be a brat. It really depends on what CORE personality you want your main character to have, how you want their surroundings to affect them, and how you want their character to grow. Do you want your young character to go from a brat to an awesome character that I like? If so, go for it, but it'll be a challenge. 