Six out of 39 businesses have sold knives or blades to children under 18 in test purchases carried out across Medway, the council has said.
Medway Council said items sold to under-age decoys included two large kitchen knives, a serrated plaster saw and three Stanley knives.
The test purchases were carried out by trading standards officers at the end of July.
The council has urged staff at shops selling knives always to check ID.
Councillor Mike O'Brien said: "We are fortunate in Medway that we don't have the same kind of issue with knife crime as many other parts of the UK, but it is vital that shops play their part in ensuring it stays that way.
"The six shops where knives were sold failed to ask for identification with one staff member believing it was fine to sell such items to anyone over 16."
Businesses visited included discount stores, independent hardware outlets, supermarkets and DIY chains.