Donald E. Westlake 
Murder Among Children 

When some West Village hippie kids get mixed up with the mob, the results are murder—and ex-cop Mitch Tobin tries to clean up the mess.  On the outskirts of Greenwich Village, among a wasteland of warehouses and tenements, a group of young people are opening a coffee shop. They are idealistic, giddy, and beautiful—the picture of 1960s youth—but their optimism cannot last. When a corrupt detective comes around demanding regular bribes, one of the young hippies, Robin Kennely, asks for help from a distant relative, the honest but fallen former cop Mitchell Tobin. When Tobin visits the coffee shop for the first time, he finds Robin in a state of shock, clutching a knife and covered in blood. Two corpses lie upstairs. It seems impossible that anyone but Robin could have killed them, but for the sake of a group of children whose lives are so much brighter than his own, Tobin attempts to prove otherwise.