Patricia,
the beauty of inquiry is really when the children take ownership, as you have described. When one child is vigorously arguing their case, you know that they are invested and interested.
Letting the science come through means that you don’t need to be an expert on the topic – and indeed, I often find an air of slight befuddlement serves me well. I’ll say something like: “I’ve seen this with another class, but they did XX and you are doing YY so I’m not sure how it will turn out — what do you think will happen?…”