Parents are being warned about the risks of giving their children alcohol to take to parties or drink on the street.
Devon and Cornwall Police are using this year's alcohol awareness week to highlight so-called "parent-pushing".
Buying alcohol for a child aged under 18, other than for responsible home use, can carry a maximum £5,000 fine.
Sgt Mark Harper, of Devon and Cornwall Police, said it was "irresponsible and should not happen".
Mick Read, a security guard at the Nisa convenience store in Barnstaple, said: "Children seem to be drinking younger and younger.
"Earlier in the year there was a 10-year-old who could barely stand outside here.
"His mother came along and picked him up and had a laugh about it."
Sgt Harper, who is based in the town, said: "There are occasions when we come across people who say 'we got this from home, our parents gave it to us to take to the party'.
"That is just irresponsible and should not happen."
An undercover operation by Devon County Council's trading standards department and Devon and Cornwall Police found that 34% of the off-licences and pubs visited sold alcohol to under 16s.
28 off-licences and 23 pubs in the mid-Devon area were targeted during August and September.
Councillor Roger Croad, Devon County Council cabinet member responsible for trading standards, said: "It shows the challenges ahead when one out of four retail premises and one out of every two pubs checked sold alcohol to children, when simply asking for proof of age would have prevented the sale."