The Spiderwick Chronicles: Nixie's Song Book 6 by Tony DiTerlizzi

The Spiderwick Chronicles leaves the old-fashioned charm of New England far behind and heads south for some fiendish faerie fun in the hot Florida sun. Eleven-year-old Nicholas Vargas only thinks his life has been turned upside down after his developer father remarries and moves his new wife and daughter into the soon-to-be completed Mangrove Hollow.

But an expedition to a nearby lake turns up a little nixie with a giant problem—the huge, lumbering, fire-breathing variety—and it’s up to Nick; his stepsister, Laurie; and his big brother, Julian to figure out the best way to stop a host of rampaging giants before all of Florida goes up in smoke.