Dead Heat (Alpha and Omega Novels, 4) [Briggs, Patricia]

For once, mated werewolves Charles Cornick and Anna Lantham are not traveling because of Charles's role as his father's enforcer. This time, their trip to Arizona is purely personal. Or at least their visit starts out that way...

Charles and Anna soon discover that a dangerous fae being is on the loose, replacing human children with simulacrums. The fae have started a cold war with humanity that's about to heat up--and Charles and Anna are in the crossfire.