A generally interesting book that sadly falls a little short of the mark set by the Artemis Fowl series. This review contains minor spoilers.
The plot moves at a steady pace and is suitably action-packed, but relies a little heavily on unlikely circumstance - the pre-teen children of a former criminal mastermind left unsupervised for an unspecified amount of time. The villains fall short of the usual standard for Colfer's villains, and the quirky novelty of the villains' methods removes any impression that the protagonists are ever in real danger - even when one of them is literally murdered.
The "nunterrogator" was particularly disappointing, falling squarely into the racially insensitive trope of "spanish character who sprinkles their english dialogue with random spanish words for no reason". This is the kind of trope one expects to see in mediocre American crime shows, not the follow-up to the magnificent Artemis Fowl saga. The other villain comes closer to appearing as a credible threat, but still lacks the menace of classic Fowl villains like Turnball Root, Opal Koboi, and Artemis Fowl jr himself.
Colfer also seems to lose track of his own timeline, stating that five years have passed since the events of the Last Guardian, but ageing the Fowl Twins by seven years in that time period. Perhaps a future volume will explain this away - time travel is very much an option in Colfer's world - or perhaps it is just an oversight on the author's part.
Overall the Fowl Twins earns three stars. One might argue that it is aimed at a young audience and shouldn't have menacing villains or real danger, but the original Artemis Fowl saga was aimed at young audiences as well, and never shied away from frightening content.