Police were contacted about the incident which occurred at about 7.30pm on Monday (June 13) at Hainsworth Park in Farsley.
A scene is in place in the park to undergo forensic examination.
Police say the park would have been busy at the time and officers would like to hear from anyone who witnessed any part of the incident or who has any information that could assist the investigation.
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Anyone with information is asked to contact Leeds District Safeguarding Unit via 101 quoting crime reference 13220321683 or online via: westyorkshire.police/101livechat .