Henderson's Boys is another series of young adult spy novels written by Robert Muchamore, this time about the beginning of CHERUB.

Muchamore writes on his website:

"The world of Henderson's Boys is rougher and more dangerous than CHERUB. Instead of drug dealers and terrorists the enemy is a brutal Nazi regime that will torture and kill young agents if they're captured. I hope you enjoy meeting Charles Henderson and the first CHERUB agents!"

https://www.muchamore.com/hendersons-boys