What literary device/term is this? "To die would be an awfully big adventure."

Solution 1:

I agree with Zan700: there's no definite literary device. It doesn't LIKEN death to an awfully big adventure. It is simply Peter Pan's understanding of what death IS. It's an adventurous child's understanding. And perhaps it makes adults who read the book wonder if their understanding of death is any more accurate, or useful, than his.

You probably know J.M.Barrie's own son died while still a child. Perhaps that's why he gave Peter that line. And he donated the royalties from Peter Pan to the Great Ormond Street children's hospital in London, which still benefits from his gift. Patients in the poorest health there might be reassured by the idea of embarking on an awfully big adventure.