The allege sexual assault was at a café on Northenden Road between 11am and 11.50am on Saturday.
A GMP spokeperson said a child was approached at the counter while they were waiting with a parent.
A woman in her 30s was arrested but police are trying to tack down the victim and any witnesses according to an appeal today, Wednesday.
A GMP spokeperson said: “We are in a position to ask for the public’s assistance as we appeal for the child and their parent to speak to us.”
Anyone can contact police on 101 or the LiveChat on gmp.police.uk.
They can contact the crime charity Crimestoppers on 0800 555 111.
This article was written by Jack Tooth. To contact him, email [email protected] or follow @JTRTooth on Twitter.