Guernsey Police said the incident happened on Thursday 6 June, some time between 10:30 and 11:30 in the field on Guelles Road, which is adjacent to Amherst Primary School.
Police especially want to speak to the potential witnesses described above, but anyone else with any information is asked to contact Guernsey Police on 725111 or report it anonymously to Crimestoppers on 0800 555 111.