Q: A 12-year-old died while practising for a motocross race. It's already sad enough for his parents, and people are saying things like "the kid and his parents must have known how dangerous it is, so they must have been prepared for this kinda accident." and I am like WTF? What kind of parents are prepared for their own kid's death? How could these people become so indifferent to other people's feelings? How could they even hold the kid accountable for his tragic death? He's only 12 years old. この表現は自然ですか?
A: just some little things to fix:

...saying things like, "The kid... this kind of accident." And I'm like, WTF?
Q: the motocross is a physically demanding sport that takes place in all kinds of conditions.

"El motocross es un deporte físicamente exigente, que toma lugar en cualquier tipo de condiciones" この表現は自然ですか?
A: It's perfect
Q: I want to motocross
I want to go motocrossing
A: I want to race motocross.
I want to do motocross.