How did Robert Muchamore come up with the name CHERUB in his books?

The name CHERUB is an acronym that stands for "CHildren's Emergency REsponse Unit and Base." It was inspired by the real-life British spy agency MI5, which has a program called the "Young Intelligence Network." The books in the CHERUB series revolve around a secret service organization that recruits young people to train and undertake missions for the government.