A 16-year-old has been arrested following the report of the incident, which happened at 4.15pm yesterrday.
Lincolnshire Police are appealling for anyone who may have seen something at that time to contact them
A spokesperson said: "We would like to speak to anyone who may have seen anything or anyone acting in a lewd manner near by the main entrance to the park.
"We would like to speak in particular to a man and woman who spoke to a member of park staff at the refreshment kiosk. The couple were pushing a buggy. If you were in the area and think you may have information that will help our investigation please get in touch."
A 16 year old man has been arrested for an offence under the sexual offences act and released under investigation, police said.
There are a number of ways you can contact the police with information:
• By clicking on the email link [email protected] - please remember to put the reference 298 of 22/9 in the subject box.
• Via our non-emergency number 101, quoting reference 298 of 22/9
• Through the independent charity Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org.