The incident happened as the girls were walking along Princess Way, Wellingborough, between 3pm and 5pm on Sunday (February 24).
A police spokeswoman said: "They were approached by two boys, thought to be aged between six and 10 years old. One of the boys produced a small pen knife or Stanley knife and held it towards the girls and asked them to guess his age.
"As the girls tried to walk away, the boys chased them, pushed one of them against a car and held a knife to the others face. The girls then managed to run away."
The boys were both white, aged 6 to 10 years old, small, with skinny builds, and short mousey hair.
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Witnesses to the incident, or anyone with information, are asked to contact Northamptonshire Police. Alternatively, they can speak to Crimestoppers anonymously on 0800 555111.