I think that the answer you are looking for comes at almost the very end of the chapter. I think that your answer should be that the littleuns see snakes in the fire.
The kids as a group see lots of things in the fire. Or at least, the fire is described in many ways. It is described as a squirrel climbing a tree. It is described as a jaguar creeping along.
But the one time that the littleuns describe what they see, they say that they see snakes. This happens just before they realize that the littlun with the birthmark is missing.