In our experiments with fire last summer, we always started by getting a bucket of water.
I was actually impressed with their snow solution. The snow was icy, slushy; not powdery, light. DD put out their first experimental fire, then DS put out the second fire with it. I examined the scene, removed all remaining fuel, asked them what made them think it was out, and how they felt they could be sure. They had good answers. We also had rain earlier in the day, and everything around was wet.
I will continue to debrief DS. I think the "what if" method is best for getting him thinking about how things can get out of control; showing him the limitations of what he or I could do if it does; knowing how to keep it in control (limit fuel and air; put it out); and other ways to solve the initial problem of wanting to stay outside, but be warm, like KEEPING YOUR HAT ON!
I've also decided to de-emphasize fire fascination by spending some serious time with them doing other (non flame) experiments as well as working on map & compass skills; knots; and perhaps shelter building.