Unlike in the Federation universe where Vega IX was colonized sometime before the 2150s and was a major United Earth settlement, mirror Vega IX was colonized later, in 2230, and was a far more desolate and backwater of a world. The empire's Bureau of Interstellar Hegemony chose Vega IX for strategic reasons even though its barren Class M surface offered colonists little in the way of resources.
In 2265 a meteor crashed on the planet near the main Terran settlement of Orbis. The meteorite had strange properties and gave the preadolescent children the power of suggestion over their parents and other adult blood relatives. The children organized themselves into a group, and, through their 10 year old ringleader, Jason Dukes, demanded that their parents leave the colony for someplace more fun. Although the parents did apply to leave the colony en masse, the bureau passed a law forbidding any colonists from leaving Vega IX.
Unable to leave, the colonists then staged a coup and declared themselves independent from the empire. James T. Kirk responded to the crisis and his security forces were easily able to round up all of the conspirators. Kirk eventually realized that it was the children who had prompted their parents to stage the coup. To teach them a lesson, he had 5,000 parents executed and then recommended that imperial administrators round up the psychic children and ship them off to Vulcan for study and possible training as intelligence agents or even assassins.
The shuttle carrying the children veered off course and was never seen again, although there were several reports of the lost children being sited on Vega following their exile to Vulcan. (TOS episode: "Mirror, Mirror"; Decipher RPG module: Through a Glass, Darkly)