* Amy Cahill: The protagonist, a young girl who becomes the central figure in the quest for the 39 clues.
* Dan Cahill: Amy's older brother, who is also a key player in the search for the clues.
There is no mention of Gideon and Olivia Cahill having five children in the 39 Clues series. The books focus on the descendants of a much larger family tree, with hundreds of descendants.
It's possible you're thinking of a fan-made story or are confusing the Cahill family with another fictional family.
If you're looking for more information about the Cahill family, you can find a detailed family tree on the official 39 Clues website or on fan wikis dedicated to the series.