The LaFarges must have the best pension plan ever, since they've retired to Mars at 55 and 60 years old (14). (Fun fact: you probably assumed that Lafe is 60 and Anna 55, but it's actually the other way around.) On the other hand, they did lose their 14-year-old son to pneumonia (76), so they probably could use a break. And that's why, Anna says, they've come to Mars: "to enjoy our old age in peace, not to think of Tom. He's been dead so long now, we should try to forget him and everything on Earth" (11).
Like all married couples, they have slightly different reactions to things—like the sudden reappearance of their dead son. Lafe is excited, while Anna is worried by Tom: "Lock the door and come to bed," she says. "I won't have anything to do with it."
But Lafe—like all of Bradbury's protagonists—is more open and curious than afraid. He calls out to the small boy standing out in the rain, saying, "Tom, if that's you, if by some chance it is you, Tom, I'll leave the door unlatched. And if you're cold and want to come in to warm yourself, just come in later and lie by the hearth; there's some fur rugs there."
By inviting Tom in, Lafe shows that he's like "Pop" from "Night Meeting," or Spender (although less murderous). He wants to experience all the strange new things that Mars has to offer, even if he's cautious at first. (A note on names: "Lafe" is probably short for LaFarge, but if so, we never learn the guy's first name.)
Ironically, it's Anna's acceptance of Tom as her son that ends up messing up the situation. She thinks things are back to normal, so she's the one who pushes for the trip into town that ends up confusing Tom and destroying him. In a way, we could say that her problem is that she takes Tom for granted. Like all the antagonists—Biggs, Father Stone—she's not interested in what makes Mars different. She wants it to be just like Earth.