It's set in a strange, hybridized alternate world where computers arrived on the scene much later than they did in our timeline, and then, inexplicably (and mysteriously) failed, plunging the world into a dark age from which a kind of dictatorial dystopia has emerged.
The new dictatorship is based upon the "New Age of Exploration," a strange golden age in which new, unmapped and unsuspected lands are regularly discovered by explorers who find that the old, computer-generated maps are wrong, omitting vast lands. These lands are ripe with natural resources for plundering, and if the occasional indigenous population gets in the way, well, that's nothing a few jackboots can't take care of.
Kit, MK, and Zander are the children of Alexander West, a storied explorer who is missing and presumed dead. Though they are orphans, they have been left to fend for themselves in the rambling old house in the explorers-only neighborhood where they were raised.
But when Kit receives a mysterious book and it leads him to half a treasure map, the chase is on. The children's father has sent them on a kind of high-stakes scavenger hunt from beyond the grave, and the secret police and the establishment they represent are keely interested in the outcome, which may or may not be a treasure-trove of ancient Spanish gold.
http://boingboing.net/2013/08/20/the-ex ... -trea.html
Amazon UK doesn't have it in stock yet, so I've ordered my copy from Amazon US
p.s. note that the cover image has 4 kids exploring together