IMHO, although not the question, the most important element is the driver and their driving skills.
"pre-school to primary grade child" is "under eight years old" - so neither the nine-year old or the twelve-year-old are covered by the specific child regulations.
With an auto-sensor, the truck should have an indicator when the passenger air bag is disabled, shouldn't be a guessing game.
My brief research of "the law":
https://www.ontario.ca/laws/statute/90h08
HTA 106 (4)
4) No person shall drive on a highway a motor vehicle in which there is a passenger who is under 16 years old unless,
(b) that passenger is required by the regulations to be secured by a child seating system or child restraint system, and is so secured. 2006, c. 25, s. 1.
https://www.ontario.ca/laws/regulation/900613
O.REG 613: SEAT BELT ASSEMBLIES
8. (1) Passengers under eight years old are classified as follows for the purposes of this section:
3. Children weighing 18 kilograms or more but less than 36 kilograms and who are less than 145 centimetres tall are classified as pre-school to primary grade children. O. Reg. 195/05, s. 1.
(7) A pre-school to primary grade child shall be secured,
(a) if there is a seating position in the motor vehicle that has a seat belt assembly consisting of a pelvic restraint and a torso restraint, in that position,
(i) on a child booster seat that is used in the manner recommended by its manufacturer and that conforms to,
(8) Despite subsections (7) and (7.1), a pre-school to primary grade child shall not be secured in a seating position if the seating position has a front air bag that has not been turned off or cannot be turned off. O. Reg. 195/05, s. 1; O. Reg. 236/09, s. 2 (9).