The Recruit by Robert Muchamore (Cherub #1)

A terrorist doesn't let strangers into her flat because they might be undercover police or intelligence agents, but her children bring their mates home and they run all over the place.
The terrorist doesn't know that one of these kids has bugged every room in her house, made copies of all her computer files and stolen her address book. This kid works for CHERUB.
CHERUB agents are aged between ten and seventeen. They live in the real world, slipping under adult radar and getting information that sends criminals and terrorists to jail.
For official purposes, these children do not exist.