Carmen and Juni Cortez are spies with the OSS.
When we first meet them, Carmen is a 12 years old girl and Juni is a 9 year old boy.
Both are the children of Ingrid and Gregorio Cortez, long-time operatives for rival organizations who met while on the job, fell in love, and decided they would prefer to start a family rather than remain out in the field. Since the work they did was so dangerous and their desire to protect their offspring so strong, they chose to not tell the kids anything about what they once did.
Which is why when the two disappear suddenly while on the first mission in nearly a decade (let's face it, they were both itching to get back out there!), Carmen and Juni were stunned when bad guys started flying through the windows and smashing the doors and otherwise find something hidden in their house.
It will not take too long for them to get into the spirit of the rescue and boldly head out to find their captive parents and save the day, not only rescuing Mom and Dad but also preventing a whole lot of young kids from being replaced by evil robots replicas.
From that exciting adventure, the two became members of OSS, the organization that was running their parents (okay, at the beginning we are told Indrid and Gregorio worked for separate agencies, now, not so much).
Note also that while the explanation of the meaning of 'OSS' is not given in the movies, in one of the books it is said to be the "Organization of Super Spies". Apparently it is based in the United States but is an international spy organization dedicated to countering the evil enemies around the globe. Eventually the OSS and its junior program, using kids as the go-to operatives, will be run by Carmen and Juni.