A knee airbag installed in a lower portion of an instrument panel has a deployed condition wherein it extends rearward and upward. The airbag has a left lobe, a right lobe, and a central portion between and joined to the left and right lobes. A vent is formed in the reaction surface of each of the lobes to allow inflation gasses to escape from the airbag when, during a crash event, an occupant force applied forwardly against the lobes is below a threshold level. When the occupant force reaches or exceeds the threshold level, the vents are urged against the instrument panel to substantially block the escape of inflation gasses. The threshold level is selected such that a child-sized occupant standing forward of a desired passenger seating position exerts less than the threshold level.