Personally, I think the Fab 5 are far more worthy of the VK title than the Sea 3, Core 4, Dizzy, Celia, and other Isle kids. Because the Isle kids are the children of villains, but that doesn't make them villain children, as in child villains, it makes them children of villains. Imagine if we called everyone with a parent in jail a criminal kid instead of a kid with an incarcerated parent. They don't deserve that. The Isle kids aren't villain kids, they're kids from the Isle. And yeah, they cause trouble, but it's not really by choice. Those are the circumstances created by their upbringing and considering how minor their actions and crimes really are, I don't think faulting them for that is fair.
But the Five? They have made the choice to be villains, they want to be villains, that is what they are doing and going to do with their lives (villain and hero are extremely unfair oversimplifications and generalizations of the incredibly broad spectrum of morality but I digress.) They are villains, and they are children who are villains. That's why they're villain kids