Let's suppose that the raft in question had a responsible adult on it, specifically in charge of all the children on the raft.
Let's also suppose that the adult saw all of the relevant warning signs, and repeatedly made sure that every child understood not to stand up.
Even with all of that in place, this accident could still happen.
Would there be any blame to attribute?
I personally feel that, in this context, an 11-year-old cannot be assumed to be acting responsibly at all times - it's a ride specifically designed to be boisterous and unpredictable, and expecting an 11-year-old to appreciate that is wrong.
So for me, as a parent, a policy for a rapids ride that allows 11-year-olds onto it and then passes them full responsibly for their own safety is asking for an accident at some point.
I say this with full hindsight, as we took our kids onto Piraña at Efteling last year (4 and 5); I dithered like mad beforehand about whether to do it, even though I knew it was a relatively tame rapids, and crapped myself all around it. (They loved it obvs). I manically drilled them to stay seated, and was ready to pounce at any moment. Was I irresponsible? My honest answer is yes.
As someone who very nearly fell off Grand Canyon Rapids at Portaventura by being bloody stupid and standing to dodge a wave (THIS close), I also appreciate that stupidity cannot be prevented.
But I'm an adult. 11-year-olds should never die on a ride at a theme park due to the very nature of the ride. End of.
For me, Intamin shoulder some blame here. That may be an unpopular opinion, but I think the manufacturer should be removing the risk of death of 11-year-olds on their rides.
(But at what age should the threshold be set is an interesting question).
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