A 16-year-old boy was attacked on Hove seafront as he sat on steps at the King Alfred amusement park.
PC Natasha Swan said: “There were a lot of young people in the area at the time of the incident.
“We are particularly keen to speak to a woman who was walking past and was heard saying: ‘You can’t do that to children.’
“If you are that woman or you know who she is, then please get in touch.
The boy was attacked on the steps leading down to the trampolines at the King Alfred when a man he didn’t know pushed him.
The assault happened between 7.30pm and 8pm on Tuesday 9 July.
Sussex Police said that the victim had short brown hair, a medium build and was wearing a grey polo shirt and blue three-quarter-length shorts.
Anyone with information that could help officers with their inquiries is asked to call Sussex Police on 101 or the independent charity Crimestoppers anonymously on 0800 555111.