Oh wow, that is scary!
I was once on a ride at Worlds of Fun in Kansas City called the Timberwolf rollercoaster. My mother and I got in the cart, in the front and as it was about to go over the first hill our seat belt came unfastened! So it was a ride full of terror rather than fun for us as at the time it was advertised to be the fastest wooden roller coaster in the world. I still remember my mother pushing me down under her as we tried to hang on for dear life.
We weren't injured, but the clerk and the park manager got a good chewing out by us. My mother considered to sue, but decided not to as they promptly installed shoulder guards into the rides just a few days after.
So I can imagine how scared those kids must have been. My thoughts are with them!