Something similar happened with my DC when she was 7.
Parent had bowling party for child in town straight after school finished, which was about 1 mile from their school.
I offered to help drive them there and stay in the background then help drive them back to the school where parents were picking up from (as I lived so far away it wasn't worth going home in between).
She said both parents would be driving so no worries.
I even specifically asked about making sure seatbelts were on properly, etc ("you know what kids are like!"), and was assured they'd be safe.
Turns out they only used 1 car for about 8 children and on the way there she put her own dc in seatbelts and the rest in the boot (estate), and in the footwells!
My DC was very adamant that it wasn't safe and she needed a seatbelt but the mum convinced her by saying it was only around the corner and they didn't want to be late, so she went along with it.
Once the party was finished I had a text telling me she was refusing to get into their car and could I collect her, which I did.
She told me why on the way home and says they were trying to do the same thing.
I was so proud of her for having the courage to refuse to put herself in danger again, and absolutely fuming at the mum.
I ended up confronting her the next day and saying how disgusted I was that she'd put the safety of our children at risk.
She honestly couldn't see the problem though as it was 'only a mile' and thought I was being over protective.
Needless to say her teen dd is now on insta, etc, often with inappropriately dressed photos for one so young.
From my point of view if I was in a similar position (and it would have had to be no other choice/forced) it would've been my own dc without seatbelts, not those who'd been entrusted into my care by others!
Although, it wouldn't have happened as I would've asked for help, paid for a taxi or walked rather than take that risk!