Crew cabs are not available with an Air Bag switch. I've looked into this and the general consensus seems to be that since kids are safer in the back seat, Ford doesn't want to encourage drivers to use the front seat when a back seat exists. SuperCab models have the switch.
BTW, NO child is old enough to sit in the front of a vehicle with an airbag. Even at 10 years or older there is considerable impact from the airbag on a body. Keep the kids in the back where they belong and keep 'em safe. They make SUVs for family's like your's and mine where we have 4 or more children. It's just a fact of life.