It is called The Bitter Seed of Magic by Suzanne McLeod. Apparently, it is the third book in her Spellcrackers.com series. It is indeed an urban fantasy with, among others, fae and vampires. The main character, Genny Taylor, is sidhe fae and one of her friends, Tavish, is a kelpie. The plot in this particular book does revolve around faelings (half-human/half-fae) disappearing.
Suzanne McLeod - Books:
On the surface, Genny's life seems ripple-free right now. Finn, her sexy boss and -- well, Genny's not sure what else she wants him to be -- has stopped pushing for a decision on their relationship. The seductive vampire Malik al-Khan has vanished back into the shadows. And the witches have declared her no longer a threat. But unless Genny can find a way to break the fertility curse afflicting London's fae, she knows this is just the lull before the magical storm.
Then a faeling -- a teenage girl -- is fished out of the River Thames, dead and bound with magic, and Genny is called into investigate. As she digs through the clues, her search takes a sinister and dangerous turn, exposing age-old secrets that might be better left buried. Then another faeling disappears, and Genny finds herself in a race against time to save the faeling and stop the curse from claiming its next victim -- herself!