A man approached two young children and struck one of them on the forehead this morning, causing a red mark.
The incident happened in Dulley Avenue, Wellingborough, between 8.40am and 8.50am.
The offender was a tall black man wearing a black scarf and dark clothing.
Police are asking anyone who was in the area around the time of the incident and may have witnessed it or have information about it to contact them on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.