CPS is enforcing the law. According to Maryland law children under 8 require supervision. The supervisor must be at least 13.
Don't blame CPS for doing their job and enforcing the law.
Not sure if you are intentionally misreading the law or not, but that law states that the child under 8 can not be left alone in a home or car. It is perfectly legal (yet still up to the parent to decide) to allow a child under 8 play outside alone.