Molgrips, it’s a tricky question to say if the childs head will land in the correct area of the Airbag. Also the positioning of the diagonal strap will have a major bearing on where the head lands in the airbag.
Obviously, if the childs head lands on the edge of the airbag then it will be subject to a rotational force. Not something that is likely to have a good outcome.
The question of whether the childs head will actually move forwards enough to reach the airbag depends on several factors such as decelerative force, slack in the seatbelt and the individual design of the car etc.
From personal experience in motorsport i can say that even with 6 point harnesses, fully tightened, it is scary how far the head moves forwards in an impact.
Perhaps we need to fit all kids with a HANS system ? Not really practical though.
US airbags are generally larger than european ones. This is a tacit admission by US manufacturers/regulators that many of the owners don’t wear a belt at all !