Diana Halvorsen is the four-year old daughter of Ralph Halvorsen, the moon administrator. She's hardly in the novel; she walks on for just a page or two. But she's symbolically important. She was born on the moon, which means that she's the first space baby in the book. In the light gravity, kids grow fast—"But," Halvorsen says, "they don't age so quickly—they'll live longer than we do." (10.27)
Diana parallels the space baby that Bowman will become—she's a creature of the stars, and she lives longer (though she isn't immortal, like Bowman.) And as with Bowman, humans evolving in space is seen as pretty darn cool; the novel dwells on her graceful carriage" and "unusually delicate bone structure." (10.28) She is, Floyd thinks a "a great hope" of a future when humans will be creatures of space. (10.31)
Though Bowman and Diana are parallel in certain ways, there is one crucial difference between them—Diana is a girl. There are hardly any women in the novel (see Theme: Women and Femininity), and what women there are tend to be associated with staying at home or entertaining the men, not with exploration and space travel. In the passage about Diana, earth is even figured as a woman who is left behind:
The time was fast approaching when Earth, like all mothers, must say farewell to her children. (10.31)
Diana is sort of an exception—but only sort of. When Heywood Floyd asks her if she wants to go to Earth ever, she says no way; she thinks earth is too crowded and unpleasant. Even though she's a creature of space, she's still oddly unadventurous. In the narrative, she spends most of her time distracting her dad from the important business at hand, just as other women in the chapter (like the stewardess on Heywood's plane, or the woman reporter) try to distract the guys from the manly task of investigating the aliens. Diana symbolizes progress, but some space babies seem like they get to be more adventurous than others.