I think the PP was talking about the 4yo who sits in the front. Probably they are sitting on a booster seat, which is perfectly legal.
Yes I think it's illegal to put children in the front in some countries, but it's not illegal in the UK.
"Bracing" definitely a myth but that screenshot somebody sent mentioning bracing didn't look like it came from a reputable site so I would ignore it. The reason they generally say airbags are dangerous for kids under 12 is the same as why we say you need to use booster seats for kids under 12 - it's to do with height and at 150cm this child wouldn't need a booster seat anywhere in the world. Seatbelts are designed for people 150cm+ (which does indeed pose a problem for women who are shorter. If you're shorter, wear a seatbelt anyway. It will still do something.) Nothing magical is going to happen on the eve of her 12th birthday that makes her better at withstanding airbags. It's just an easier message to say "over 12 vs under 12" - rather than complicating it with things like if your child is 150cm or not. There is going to be higher risk for younger children and lower risk for older ones on a sliding scale. There's really not that much difference between an 11yo and 13yo for example, but an 8yo would be at more risk and a 6yo even more so. But age is an easy metric - you don't always know your child's exact height or weight but you tend to know their age in years.
The middle seat thing is out of date, it used to be the case that the middle seat was safest because side impact protection in cars has only been a thing for approx the last 25 years, so 10-15 years ago anyone driving a car older than about 10 years was at high risk from side impact crashes and the middle seat is the furthest point from that type of crash. Nowadays that is less relevant because cars have side impact protection built in so the sides are less risky, and very few people are driving around in 20+ year old cars. If you're linking/reading studies and they are based on crashes that occurred in the 90s, bear this in mind too. Modern cars have completely unrecognisable safety features compared with cars from the 80s and 90s. It might be that the back seat/front seat
I don't know if you can extrapolate NCAP data because they don't put adult dummies in the back and test them so they aren't judging by the same criteria and the score may reflect that. For instance they get marked down for not having a built in booster seat in the back even though a separate high backed booster gives better crash protection in lab tests. But certainly several experts in Sweden seem to think the front seat is safe (note here they are talking about deactivated airbags because they are discussing use with rear facing child seats and not adult sized children) www.carseat.se/rearfacing/position-in-vehicle/front-seat-safety/
And if your car has a good NCAP rating for front seat occupants, it's not a bad place to put an adult-sized person.
I’m also interested in the answer to the airbag question. My 8yo quite often goes in the front but I haven’t been turning off the airbag, now wondering if I should?
Check the car's manual. For forward facing children in car seats you will have an instruction in there. Some cars say not at all, some cars say airbag on, some say airbag off. All say seat back as far as it can go, I believe. If your 8yo is not on a booster seat you'd be best off using at least a backless booster if not a full high back until 150cm or until the belt fits over the hips when he's bending his knees over the seat edge - for most 8yos, bending their knees over the seat edge (as is comfortable) tends to bring their bum and hips forward, which puts the lap belt higher over their tummy.
One issue that hasn't been mentioned in the thread is the smartphone problem - recently more head injuries seen in children who pre-crash were positioned bent over looking at a smartphone. When they're in this position they are at risk from the airbag as their head is more in the place that it's likely to explode. But if you want her in the front so you can chat, then this is less likely.
Sorry OP, I bet you wish you'd never asked 