The assault occurrend on Old Lane, Ovenden, near to a disused mill at 3.45pm on Saturday May 12.
Stones were throw from the mill which hit a child on the head, causing slight injury.
Officers are trying to find a suspect described as white female with long dark hair down her back, 15 years of age, chubby, approx 5ft 4ins tall. wearing blue jeans, dark blue coat with fur round the hood.
Another suspect is described as a white male 6ft tall, skinny build, approx 15 years of age. wearing green tracksuit with hooded top
A spokesperson for the Halifax Neighbourhood Policing Team said: "If you witnessed this assault or have any information that could assist, please call 101, use the 101 Live Chat on our website or contact Crimestoppers on 0800 555 111 and quote crime reference 13180227897."