"How do you help your kid understand that the world you brought them into is quickly collapsing?"
I'd say as a parent it would be pretty irresponsible to say anything that would make the kid feel any sudden anxiety.
We don't really talk about it. If our kids express their worry the we will say the truth - biomes are not doing too good. But we don't really consider it as some major doomsday looming over.
Personally I was really anxious as a kid. I was really scared when I learned that one day sun will turn to red giant and destroy earth. Ditto about asteroids. The Amazon (30 years ago as kid I refused to eat at McDonalds for a while). And there were also actual bad things happening to people close to me - but you get the point. Easily distressed.
As an adult, I see no way this existential anxiety would have done any good to me.
So, as a parent I play it cool. And besides, we don't know what is going to happen. It's not going to be rainbows and sunshine ... but really. If you were with a kid on a train to Auschwitz, would you tell them me and him would soon be gassed to death, or lie to them it's all going to be fine. I'd lie.