At first I thought the 'her' in your question must be in reference to the big sow that the boys kill in chapter 8. After flipping through all the sections of the novel where the boys act out the chant, I discovered that the boys originally use 'her' as a pronoun in their first chant after they kill the pig in chapter four. The chant appears two more times after that in the novel. The second time occurs when the boys re-enact a hunt after they run into the boar in chapter seven.
In chapter seven, the boys unsuccesfully try to bring down a wild boar. Later Robert pretends to be the boar, and the chant changes to 'him' to suit the appropriate gender of their target. In this particular scene, the game becomes much more rough, violent even, with Jack even trying to jab Robert with his spear; the violence of their pretend killing dance in combination with their lethal chant foreshadows that the next game will be much more deadly.
In chapter nine, the boys participate in their final, most lethal 'dance;' on this occasion the chant uses the masculine possessive pronoun 'his,' and Simon pays a terrible price for their brutality.