In chapter 2, the boys decide to climb to the top of the mountain and attempt to build a signal fire. After gathering a massive amount of dry wood from the surrounding forest, Jack takes Piggy's glasses and Ralph uses them to light the fire. Once the fire is lit, the pile of dry wood bursts into flames and the wind carries the sparks across the surrounding woods, which starts a massive forest fire. As the boys contemplate the power of their fire, Piggy begins criticizing them for their impulsive behavior and lack of self-control. He then notices that the littlun with the mulberry-colored birthmark is missing, and the remaining boys turn towards the flaming forest. As a tree explodes into flames, the littluns begin screaming "Snakes! Snakes! Look at the snakes!" The older boys know that the littlun with the mulberry-colored birthmark has died in the forest fire but choose to repress the difficult thought and refuse the reality of the situation.

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.