More news at this link, including some statements from witnesses. Apparently the boy's older brother was with him.
http://www.kmbc.com/news/person-dies-at-schlitterbahn/41091622
In the video, they mention there was a problem with harnesses.
http://www.kmbc.com/news/person-dies-at-schlitterbahn/41091622
“The little boy said to a worker, 'I just saw my little brother die because of one of your attractions,'" Castaneda said. "(The boy) looked skinny. He shouldn’t have been on that ride."
Castaneda said the boy's brother then left to go find his mother who was on another ride.
Castaneda said she noticed that one of the other riders had a bloody eye, and that the park didn't immediately close.
In the video, they mention there was a problem with harnesses.