An 11-year-old girl needed hospital treatment after binging on a "vast quantity" of vodka, police said.
The girl was found drunk on a bench in Westgate Street, Gloucester, last Friday night by officers on patrol.
She was found at about 10.20pm and was described as being incapable following the consumption of a vast quantity of vodka with her friends.
The girl, from Gloucester, was treated at the city's Gloucestershire Royal Hospital and later discharged.
A Gloucestershire Police spokesman said: "Police will be taking follow-up action with partner agencies and speaking with the family about the issue.
"A 15-year-old girl who was with the child was spoken to for concealing vodka in a soft drinks bottle."
The incident happened during the second night of operation Nite-X, a project run by the special constabulary and supported by Gloucester City Council, which is tackling under-age drinking, binge drinking, alcohol-related disorders and licensing breaches.
Sergeant Liz Lovell said: "This was a shocking discovery that highlights exactly why we are running this operation.
"It is understood the girl was given the vodka by an adult who bought it from a shop, which is another issue we are tackling as part of Nite-X.
"Where shops are found to sell drink to under-age customers or adults who are buying it for children, we will issue fines and work with the business to make sure it doesn't happen again.
"One thing we will be feeding back to officers is an increasing tendency for youngsters to conceal alcohol by pouring it into soft drinks bottles."
Jane Windle-Hartshorn, from Gloucester-based charity Family Lives, said parents need to warn their children about the dangers of alcohol from a young age.
"We speak to thousands of families throughout Gloucester and the UK every year and know that issues around alcohol are a concern," she said.
"Evidence shows that parents are the main influence on how children approach alcohol and whether or not they drink to excess.
"Many parents do not talk to their children about drinking alcohol early enough, and we hope that the Government's alcohol strategy will reflect the growing body of evidence which shows that equipping parents to understand the risks of drinking to excess, and how they can talk effectively with their children about alcohol, is the best way of preventing children experimenting with alcohol at an early age and can prevent binge drinking in teenage and adult life."