When is it safe for a child to sit in the front seat given the sir bag dangers?

The only time I know of is when the car is out in a large open field surrounded by trees and grass and the battery is disconnected ensuring that the Airbag will NOT go off.

That airbag can go off at anytime. That would be the equivalent of lifting your child up 15 feet into the air and dropping them on their face. It's not very good for the child.