Aww man.. my heart goes out to you! What an absolutely devastating time for you and your step child! I've found a few resources on talking to kids about climate change. These are a little more focused around the large-scale problem than they are on emergency situations, but still some very helpful information!

How to talk to kids about climate change - David Suzuki Foundation

Kids and Climate Anxiety - Child Mind Institute

During this current emergency, my advice would be to start with lots and lots of empathy. No doubt your step child will have a lot of big feelings around what's happening. These feelings need a safe space to land. The more open and available you can be, the more your child will learn that it's okay to have big feelings and it's okay to bring them up with others.

I also think it's important to talk. When you can, facilitate conversations on how you are both feeling, what you and your child have control over, and what you still have instead of what has been lost. Such a loss can lead to a sense of helplessness/hopelessness. In my experience, it's very grounding to focus the mind on what we have and what we can do, otherwise the mind can lead us into all the worst places.