Over 20 lbs will deploy the bag. 40 lbs is the lowest weight at which one should seat a child (forward facing) in the front seat.
But your question is not really complete. You forgot HEIGHT. The air bag will deploy and be shaped for a person 5'2" or taller with anticipated impact 10-12 inches from the dash. Hence booster seats. A child of ANY weight will be injured if they do not have the weight\height to preload the shoulder harness and arrive on time at the designed bag deployment zone.
More info: http://www.safercar.gov/Air+Bags
If you do not have a switch it is because you have a backseat - where until 13 Years old they should (and in my bagged vehicles DO) ride.
But by all means - do your own research and do not rely on forum flame warriors to satisfy your need to protect your child (or justify placing them in harms way)!
J Wm Bishop EA, ASADE
The wagon should, of course, be as light as possible, but strength should not be sacrificed to lightness, for on any but the regularly traveled roads, the wagon will get many a hard knock...