If you ask most parents, they will tell you that swearing is not in their child's vocabulary. Or, if it is, it's not encouraged. Kids swearing is looked at as a sign of bad parenting or bad manners, which is why a lot of parents try to prevent it from happening. I vividly remember playing a board game with my brother when I was six years old and yelling out "DAMN IT!" My dad quickly explained to me that "damn" was a word we don't use, and to say "darn" instead.
This sentiment is shared between a lot of people. Swearing in general, not just in kids, is perceived as a sign of low intelligence. However, studies have shown that people who swear tend to have higher IQs than those who don't.
One mom, Sharon DeVellis, doesn't mind if her son swears, and it has nothing to do with intelligence.