Parents of boy killed on water slide speak for the first time about their final moments.

The parents of 10-year-old Caleb Schwab have spoken for the first time since he was tragically killed at the Schlitterbahn water park in Kansas City last August.

Scott and Michele Schwab had taken their four sons to the water park for a family day out. Caleb and his 12-year-old brother Nathan raced straight for the Verrückt water slide.

The slide, which is billed as the ‘world’s tallest slide’, stands at over 51 metres high.

The slide's name, Verrückt, means 'insane' in German.

Scott, a Kansas State representative, told Good Morning America he told the two boys to "stick together" before the ride.

"Six went to the park and five came back," Scott said.

"Before they took off I said, 'Brother's stick together'. He said, 'I know Dad'. I said, 'Look at me, brother's stick together'."

It was the last time he saw his 10-year-old son alive.

Nathan waited for his brother at the bottom of the water slide, before he starting screaming.

"He was screaming, 'He flew from Verrückt, he flew from Verrückt'," Michele said.

At the time of the incident, one witness, Esteban Castaneda, told ABC7 Eyewitness News he heard booms coming from the slide area.

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He then "saw a body wash down the slide directly after the raft did".

Castaneda said he tried to offer assistance before he saw that the boy appeared to have been decapitated.

Caleb's parents say witnesses wouldn't let them see the aftermath of the accident .

"There was a gentlemen who wouldn’t allow me to come close enough to see what was going on," Michele told Good Morning America.

"He just kept saying, 'No trust me, you don’t want to go any further'."

Scott said he asked those around him to confirm that his son had died on the slide.

"I said, 'I just need to hear you say it, is my son dead?' And he just shook his head. I said, 'I need to hear it from you, is he dead?'" Scott said.

"He said, 'Yes, your son is dead'.

"It was surreal, I don't even remember driving home."

Scott and Michele Schwab have since settled with the Schlitterbahn water park for an undisclosed amount.

The money will be used to help Caleb's three brothers.

When asked what they missed the most about their 10-year-old boy, Michele replied, "giving him hugs".

"And hearing about his day, watching him play soccer, so much, so many things," she said.