Clark County Family Court Judge William Voy sentenced the boys, who had pleaded guilty, to one count of conspiracy to commit animal cruelty in the Oct. 9 incident, the Las Vegas Sun reported Tuesday.
Witnesses said it sounded like glass was shattering, but the noise came from the boys throwing rocks at the mother cat. The two testified they left when it seemed the mother cat became angry.
The mother cat and her six kittens died, the newspaper said.
It was not clear whether the kittens died before or after being born, said Michael Watson, a prosecutor in the Clark County District Attorney's Office.
The judge sentenced each of the youngsters to have no further association with each other, to write a letter for the court on the appropriate treatment of animals, and to 80 hours of community service, 12 months of probation and a 30-day suspended sentence in the Clark County Juvenile Detention Center.
"The compassion starts at home. Parents really need to know what's going on with their children," said Stacia Newman, president of Nevada Political Action for Animals, who, along with other animal advocates, showed up for the court hearings.