Before I explain my reaction to his on-the-fly suggestion, here's some background: Chiasmus was noticed in the Book of Mormon in the 1960s by John Welch, and has since become a significant tool in appreciating some of the carefully crafted poetic passages of the Book of Mormon. The abundance of chiasmus in the Book of Mormon is, in my opinion, one of many Hebraic elements that support the plausibility of ancient origins for the text. Chiasmus is very foreign to us and is often difficult to recognize, especially since translation and modern typesetting can obscure the underlying structure. But it's a beautiful thing when recognized. And perhaps the most beautiful example known is found in Alma 36 in the Book of Mormon. But that's another story.
A problem with chiasmus is that if you're looking for it, you can find it in places where it probably wasn't intended. For example, you can find repeated words and phrases all over the place in any text, and by cherry picking can come up with inverted parallel structures. John Welch has provided guidelines to judge the chiastic nature of a passage, and these guidelines help rule out many discoveries of enthusiasts trying to spot chiastic structures. For example, it should be relatively tight in structure - not one word ever couple of paragraphs, and should show signs of being intentional (enhancing the message, etc.).
As my son suggested there might be a chiasmus in Helaman 16, I scanned the text quickly and didn't see anything interesting, and didn't recall this passage being cited as an example of chiasmus, so, while I was proud of him for having paid attention when I discussed chiasmus in the past, I was about to launch into a paraphrase of John Welch's warnings about chiastic quality to help my son understand that the few repeated words he saw were probably just coincidence and not to get too excited, etc. But first I asked again what he was seeing - and then as he pointed out the places and terms he thought were significant, it hit me: maybe he's right. And as I looked more, it seemed possible that there might be an intended chiasmus here. Not sure, but maybe. Interesting!
Below is a proposed structure, based on what my son spotted on the fly during a moment of family scripture study. The actual text from Helaman 16:1-8 then follows. The proposed chiasmus has a very logical focal point: the coming of Christ (like Alma 36, this is literally a Christ-centered passage). It begins and ends with Samuel on the walls of the city, setting the stage for his dramatic encounter with and departure from the Nephites.
Proposed Structure of a Tentative Chiasmus in Helaman 16:1-6
A. Samuel the Lamanite speaks from the walls of the city (v.1)
B. Those who believed him seek for the prophet, Nephi, and do not deny their sins, to come unto the Lord. (v. 1)
C. As many as did not believe Samuel were angry and cast stones and arrows at him. (v.2)
D. Many went away to Nephi and were baptized. (v.3)
E. Nephi was prophesying and working miracles (v. 4)
F. "That they might know" (v.4)
G. "That the Christ must shortly come." (v. 4)
G'. "Things which must shortly come" (v.5)
F'. "That they might know" (v.5)
E'. The time of the coming of various things had been made known unto them beforehand (v.5).
D'. As many as believed on the word of Samuel went forth to Nephi to be baptized (v.5)
C'. The more part did not believe Samuel and became angry when they saw they could not hit him with their stones and arrows.
B'. Those who do not believe Samuel called upon their captains to arrest him (instead of seeking out the prophet Nephi, they seek their police to arrest a prophet) and accuse him of serving the devil. (v.6)
A'. Samuel cast himself down from the wall. (v. 6)
The Text: Helaman 16: 1-8
 And now, it came to pass that there were many who heard the words of Samuel, the Lamanite, which he spake upon the walls of the city. And as many as believed on his word went forth and sought for Nephi; and when they had come forth and found him they confessed unto him their sins and denied not, desiring that they might be baptized unto the Lord.
 But as many as there were who did not believe in the words of Samuel were angry with him; and they cast stones at him upon the wall, and also many shot arrows at him as he stood upon the wall; but the Spirit of the Lord was with him, insomuch that they could not hit him with their stones neither with their arrows.
 Now when they saw that they could not hit him, there were many more who did believe on his words, insomuch that they went away unto Nephi to be baptized.
 For behold, Nephi was baptizing, and prophesying, and preaching, crying repentance unto the people, showing signs and wonders, working miracles among the people, that they might know that the Christ must shortly come --
 Telling them of things which must shortly come, that they might know and remember at the time of their coming that they had been made known unto them beforehand, to the intent that they might believe; therefore as many as believed on the words of Samuel went forth unto him to be baptized, for they came repenting and confessing their sins.
 But the more part of them did not believe in the words of Samuel; therefore when they saw that they could not hit him with their stones and their arrows, they cried unto their captains, saying: Take this fellow and bind him, for behold he hath a devil; and because of the power of the devil which is in him we cannot hit him with our stones and our arrows; therefore take him and bind him, and away with him.
 And as they went forth to lay their hands on him, behold, he did cast himself down from the wall, and did flee out of their lands, yea, even unto his own country, and began to preach and to prophesy among his own people.
 And behold, he was never heard of more among the Nephites; and thus were the affairs of the people.
Well, what do you think?
Update: A question was raised in the comments about whether this could possibly be chiasmus, since chiasmus involves short introverted phrases and not long passages. While many discussions of chiasmus, including Wikipedia's, focus on very short couplets or simple A-B-B-A patterns with a couple of words, many scholars recognize that ancient writing in the Near East sometimes had much more extensive passages of chiasmus. The existence and importance of extended chiasmus is now attested, though there was nothing in Joseph Smith's day that would have guided him in fabricating such extensive Hebraic poetry as we find, for example, in Alma 36. The leading work of scholarship on such chiasmus is Chiasmus in Antiquity, edited by John Welch (Provo, Utah: Research Press, Brigham Young University, 1981). Yes, John Welch is LDS and it's a BYU publication, but before you make the instant assumption that you can dismiss the evidence because a Mormon was involved, please note that this volume brings together the scholarship of some significant non-LDS writers, all of whom recognize the existence of ancient chiasmus that goes well beyond the very simple forms of introverted parallelism you might encounter in Wikipedia or in the relatively obscure works dealing with parallelism in the Bible that existed in Joseph Smith's day (and which were probably completely unavailable to him). The scholars writing in Chiasmus in Antiquity include:
- Dr. Joseph Fraenkel, Senior Lecturer in Hallakhic and Aggadic Literature, Hebrew University, Jerusalem.
- Dr. David Noel Freedman, Director of Program on Studies in Religion, University of Michigan, and General Editor of the Anchor Bible and Biblical Archaeologist.
- Bezalel Porten, Senior Lecturer of Hebrew and Aramaic Ancient Literature, Hebrew University, Jerusalem .
- Dr. Yehuda T. Radday, Associate Professor of Bible and Hebrew, Technion, Israel Institute of Technology, Haifa.
- Dr. Wilfred G.E. Watson, Department of Hebrew, Trinity College, University of Dublin.