The children of London are disappearing—can Is solve the mystery before she vanishes too?

Is Twite, younger sister of the daring Dido, is on a desperate mission. An uncle she never knew just showed up at the cottage she shares with her sister Penny. He is being pursued by ravening wolves, and before he dies, the uncle begs her to find his missing son—Is’s cousin, Arun.
 
The quest takes Is to London—a city mysteriously devoid of children, including the king’s only son and heir. Is soon finds herself aboard the Playland Express, a secret midnight train that leaves town once a month, just before the new moon. The kids aboard believe they’re headed to a far-off kingdom filled with fun and games. In reality, Playland is a freezing underground city ruled by the greedy “Gold Kingy,” who has lured the youngsters there under false pretenses. His real objective is to put them to work in the coal mines. But the worst is yet to come: Gold Kingy is none other than Is’s other uncle Roy! Now it’s up to Is to avoid a terrible fate and use her wits, ingenuity, and powers of telepathic communication to free the children.
 
Is Underground is the 8th book in the award-winning Wolves Chronicles, but you may enjoy reading the series in any order.