Her importance to the film is indicated in this 2011 article:
When Malick couldn’t find his 70mm epic in the editing room, he had the crazy-brilliant idea to let his 15-year-old starlet lead the way: “This was later on: They took me into a voice recording studio,” remembers Manz. “No script, nothing, I just watched the movie and rambled on . . . I dunno, they took whatever dialogue they liked.” Laid over the images, these extemporaneous monologues abut God, the Devil, and some kid named Ding Dong (“I just made that up”) gave the movie its perspective—and a surreal humor Malick never matched.
I don't know if I never knew or I just forgot that almost twenty years later the same Linda Manz was in Harmony Korine's great debut film, Gummo (1997), playing the mother, also very memorably. Here she is tapping in that:
She hasn't been in anything since 1997.
Looking closely at the first clip, if I'm not mistaken that's Stuart Margolin sitting in the background (he played Angel Martin in The Rockford Files). If so, it is a kind of continuity error, because he is in the first scene of the movie, in Chicago, playing the factory foreman that is assaulted by Richard Gere (with indeterminate consequences), and it makes no sense for him to be in this scene in Texas.
As an aside, perhaps Criterion took a little too much yellow out of Days of Heaven.
I think I need to see Out of the Blue (1980).