The boys were travelling in a Volkswagen Golf that collided with a Citroen Berlingo on the A84 between Callander and Strathyre, Stirling, at about 5.15pm yesterday.
A 54-year-old man who was driving the Golf and the 30-year-old female driver of the Berlingo were also seriously injured.
The man and the two nine-year-olds were taken to the Queen Elizabeth Hospital in Glasgow. Police said they sustained “life-threatening injuries”.
The woman is being treated in Forth Valley Royal Hospital. A two-year-old girl who was also in the Berlingo was uninjured in the crash.
Police are appealing for witnesses.
Inspector Andrew Thomson said: “This is a serious road traffic collision involving multiple casualties.
“We are now appealing for information regarding this incident. Anyone who saw this incident, or was in the area when this collision occurred, is asked to contact us.”