This quote is found in chapter 8 as Piggy listens to Ralph vent his frustrations about Jack's seeming lack of understanding regarding the importance of the fire. When Ralph admits that he is scared, Piggy looks up at him, and Ralph clarifies that he isn't so much scared of the beast as he is scared about the group's inability to recognize that the fire is crucial to their chances of being spotted by rescue groups.

Ralph doesn't want to die on the island, but Jack's group doesn't care that two people aren't capable of keeping their fire going while trying to help sustain the group through various other means. Ralph wonders what will happen if he and Piggy just give up and stop caring like everyone else. Piggy responds,

I dunno, Ralph. We just got to go on, that's all. That's what grown-ups would do.

I am using a book published by the Penguin Group, and this is near the bottom of page 139. If you are using a different printed version, start at the end of chapter 8 and flip back a few pages. It's somewhere around four pages from the end of the chapter.