What a terrible tragedy, so young.
Air bags are disabled in modern vehicles when seat belts are not buckled. The thinking is that in most crashes the exploding bag will injure worse than an unbuckled interior impact.
The manufacturers can tell in a lawsuit investigation whether or not the belts were buckled if a claimant states that they should have gone off. They scan the black box and bingo, there is the evidence. I have been involved in some of these cases when somebody is looking for an early retirement.
The bags do not go off in every crash even when the occupants are buckled properly unless the forward motion will put the occupant into the protection zone. 3/4 impacts will not trigger them unless side airbags are also in the mix.
Cars and trucks are safer now than ever, but dumb humans can still kill one another by doing dumb stuff like running red lights.
That family will be changed forever, what a shame.