In "A Perfect Day for Bananafish," Seymour's only positive relationship is with the four-to-five-year old Sybil Carpenter, who sits on the beach with the war veteran.
It's important that Seymour is friendly with Sybil because Seymour's wife's conversation with Sybil's mother and Seymour's blowup in the elevator indicate that he is not good with adults. Unlike in his interactions with adults, Seymour does not seem to feel pressure around Sybil. Perhaps it's because, unlike his wife, Sybil can't see anything wrong with Seymour. He can take his robe off in front of the girl, which is something he can't do in front of his wife because he doesn't want "a lot of fools looking at his tattoo." Although he does not actually possess a tattoo, the idea of a tattoo is suggestive of what Seymour saw during World War II? The violence of war is figuratively tattooed on his skin.
Regardless, it's clear he feels comfortable around Sybil and Sharon Lipshutz, another child, because of his inability to be comfortable around those who know how he feels after the war.