Children, for the purposes of this wikia are any character that is considered a minor in the United States of America. Basically, anyone seventeen and under, so despite the fact that they aren't generally thought of as such, most characters who are high school students would fall under this. If they appear later on at an older age, they are put under both children and adult.
In some cases, such as The Innocent, a character can be added here even if they probably aren't actually a child, so long as they look like one. If they definitely aren't a child, because their age is given, then they are not put into this category.