DALLAS - The arrest of a 12-year-old boy on charges that he molested a 4-year-old girl at a fast-food restaurant playground may point to a growing problem of sexually aggressive children, child-abuse experts say. The boy was arrested Wednesday in suburban Houston on charges of indecency with a child and aggravated sexual assault. Police would not discuss in detail what happened to the girl, other than to say her mother said she'd been touched inappropriately. The arrest came a week after the Dallas Independent School District expelled two first-grade boys after one performed a sex act on another during class....