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.
This was mentioned in the article already as the law only mentions that child's age if left home alone or in a car.
From your link:
"Under Maryland law, a child must be at least eight years old to be left alone in a house or car."
That does not fit the situation these children were in.