Toronto EMS says some of the children, who were visiting as part of a school group attending a special exhibition, fainted shortly after arriving at the AGO Friday morning.
Emergency workers said the children were conscious but were pale and suffering from abdominal pain. All were expected to recover, police told CBC.
Fire crews arrived on the scene, and the special exhibition area of the AGO was evacuated.
Fire crews told CBC that readings taken at the scene show there was no carbon monoxide present and that the air was safe to breathe.
Police said the children may have inhaled fumes from a parked truck or bus before they entered the building.