Officers were called to the scene of the incident at Fountain Lane at around 6.20pm on Thursday, 28 July.
Police have launched a murder investigation.
Officers say the area has been cordoned off and units will be at the scene for "the foreseeable future".
The young girl’s parents have been informed.
A Lincolnshire Police statement reads: "Our thoughts are with them at this incredibly difficult time.
"Her family will be supported by specialist trained officers."
Police urge anyone with information to call 101, quoting incident 419 on 28 July – or email@example.com incident 419 of 28 July in the subject line.
Alternatively, people can report via the independent charity CrimeStoppers on 0800 555 111 or online.
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