It's going to depend on the state that you are talking about. I included the fact sheet for Georgia, because it was in an easy to use format. The definition of unruly child seemed to be pretty close to that of Ohio, so there's a decent chance most states would similarly define it.
An unruly child is one that has committed a status offense according to state law. Unruly behavior can get as serious as drug abuse charges. Children alleged to be unruly can be detained, but not for more than 24 hours.