Henderson's Boys

A British spy series set in World War Two. Henderson's Boys concerns the activities of the last active British agent in Nazi-occupied France, and the children he adopts/recruits to assist him. It's sold as a prequel to the author's bestselling CHERUB Series, but has a significantly darker tone and a very different setting. Main characters include Charles Henderson, an adult British spy, Marc Kilgour, a young French orphan, Paul and Rosie Clark, who are British kids orphaned in the first book, and PT Bivott, a young French American thief that fled American for France, only to arrive in time for World War II. Other agents are added in Secret Army, including Luc, Joel, Troy, Sam and Yves.

The books try to keep things fairly realistic, and is very much in the Stale Beer category of Spy Fiction. There are no over the top supervillians that can only be defeated by bullet dodging tux wearing super spies. The series makes it clear that one mistake, one stray bullet, or one unlucky event could doom the entire group.

The series at present consists of four published books:

The Escape (February 2009)
Eagle Day (June 2009)
Secret Army (February 2010)
Grey Wolves (February 2011)

Tropes used in Henderson's Boys include:

Anyone Can Die - Including young children. Also, Word of God has said that at least one of the main characters will die before the end of the series.
Something Only They Would Say - Not exactly, but in Grey Wolves an RAF pilot tries to find out if Charles Henderson is really English by asking him things only an Englishman would know. It doesn't work, because their lack of a common social class meant that their cultural frames of reference were too different.
Brother-Sister Team - Paul and Rosie
Children Forced to Kill - First Marc Kilgour and latter many of the other agents kill Germans as part of their operations.
Child Soldiers - Because it is wartime, the role of the ERU-B is even more soldier-like than their CHERUB successors.
Cloak and Dagger - ERU-B is very much in the Cloak and Dagger tradition of British spy fiction and the Real Life World War II era British Special Operations Executive and Commandos.
The Chick - Somewhat inverted. Rosie, described on the official website as "The one who isn't a boy," is actually a tough, bossy, capable character who contributes to the team. However, the gender roles of the 1940's means that many characters persist in viewing her as The Chick.
Death From Above - Both German and British bombing raids play an important part of the stories.
Drill Sergeant Nasty - Mr. Takada, a Japanese national who is training the children as an alternative to being in a prisoner of war camp.
Everybody Was Kung-Fu Fighting - Mr. Takada is teaching martial arts to ERU-B trainees.
Free-Range Children - Rosie and Paul spend some time as free range children in The Escape. PT Bivott also spends times a free range child. The training exercise in Secret Army involves the children performing a simulated mission that involves traveling through England without adult supervision or help.
Government Agency of Fiction - ERU and ERU-B, the predecessor to CHERUB
Harmful to Minors - Children are fully exposed to the horrors of war and Nazi occupation
Historical Fiction - The series is set during World War Two. So far, all the books have taken place on the Western Front of the European Theater.
Home Base - ERU-B has a training center established by the beginning of Secret Army.
Just a Kid - The premise of ERU-B is that Those Wacky Nazis will underestimate young agents because they are Just a Kid
Kids Are Cruel - Well, not all, but several are, especially Luc
Parental Abandonment - All of the children in ERU-B are either orphans or otherwise separated from their parents.
School Uniforms Are the New Black
Spy Fiction - Charles Henderson is a British spy and begins training children to act as spies
Spy School - ERU-B trains children to be spies and saboteurs. The term "spy school" is used in Grey Wolves.
Stuff Blowing Up - It is the middle of a war after all...
Teen Superspy - The entire point of ERU-B is to use teens and even preteens as spies. Nevertheless, this is very much a Stale Beer version of the teen spy rather than something along the lines of Alex Rider or the Spy Kids movies.
The Bully - Luc, whose favorite target is Paul
The Bully Hunter - Several of the students retaliate against Luc's bullying
The Infiltration - The mission in Eagle Day and no doubt future missions
There Are No Therapists - While CHERUB averts this by providing mental health services to their young agents, World War II era Britain does not have such things high on their priority list.
The Spymaster - Charles Henderson and his superiors
Those Wacky Nazis - Including, of course, SS officers
Training from Hell - The training that ERU-B agents undergo
Your Cheating Heart - Charles Henderson is a serial adulterer to his wife. PT Bivott cheats on Rosie with a prostitute