Alex, Mattie, and Sophie Chapman are kids.
That pretty much sums them up nice and neat. They are kids. Mattie and Alex are tweens while Sophie is not there yet being around five (Alex is just a tad older than Mattie but since he got mentioned first in the previous paragraph, it seemed prudent to let her go first here.)
They are not really spies, despite the name of the adventures they have. I mean, yes, they kinda are spies, in a sense, but they do not work for any organization and have no affiliation with any organization and would have to ask permission from grownups to join or even talk with any organization so, not really spies as we think of that.
So, why are they here?
To quote the scribe for the nearly dozen adventures: "Mattie, Alex, and Sophie Chapman move from the Maryland suburbs to the Virginia countryside where their parents have opened a bed and breakfast. The old Gray Horse Inn dates back before the Revolutionary War. The kids think they will die of boredom, but they find a secret room in the tower. In an old desk they discover a magic spyglass that transports them to little-known events in American history or trips through American literature. Because the inn is near Monticello, Jeffersonian themes tie the books together."
Using this strange and extra-special spyglass, the three kids are able to travel through time and space into interesting periods of the country's history. There are even a couple of excursions that stray big time from the move-into-history paradigm and put them up against even strangers things. Can you see a giant?!
Alex, being the oldest, should be in charge [though if he really was responsible enough to be giving the orders, would he really let all three pop around into the past? I'm just asking!] but there are more than a couple of times when Mattie takes over because she is really, really smart and even if Alex might get annoyed, letting smart girls run the show is the - um - smart thing to do. Sophie just goes along because it is a lot of fun but boy can she gabber!
Are they really spies? Nah, not really. But they have neat adventures and they do have that very special spyglass.