A nine-year-old girl was assaulted in the pocket park in Park Avenue South, Abington, Northampton.
The girl was cycling in the park when two boys came over to her and asked her to leave. When she refused, one of the boys threw an object at her which hit and cut the back of her head.
The offenders are described as white and in their early teens. One was about 13 and had a round face. He was carrying a black blazer and wearing a white school top and black trousers.
The other was a little older, possibly about 15, and had brown hair. He was wearing a white school top and black trousers.
The incident happened on Thursday, 17 July at about 6.30pm.
Witnesses or anyone with information is asked to contact police on 101 or Crimestoppers anonymously on 0800 555 111.