It's best to sit higher, lower your legs naturally, and don't put them on the legs of tables and stools. This is a big taboo, because when you grow up, your bones will change slightly and you won't even notice it.
When sitting, move your ass forward, that is, move to the stool. Don't put your thighs on the stool, because they will thicken over time.
Extra-curricular activities can also play the role of exercise.