No kinship is implied between Mormon and Ammaron, but one may well have existed. Ammaron must have known Mormon’s family or he would not have spent enough time with a ten-year-old to pronounce him “a sober child, and… quick to observe.” Yahweh unquestionably had a hand in the Mormon’s selection, but it took the form of being sure Mormon and Ammaron shared a social sphere, one in which spiritual promptings were recognized and respected.
I see such kinship as plausible but argue that it must have been distant for two reasons. First, if it had been close, then Mormon would have simply been the logical heir instead of being specially selected. Amos1 gave the plates to his son, and Amos2 gave them to his brother. Sibling transmission also occurred between Nephi and Jacob, and between Amaron and Chemish (Omni 1:4–8). Later among Aztec royalty, brothers might inherit before sons. Had Mormon been a son or brother, he would have inherited the records and continued his own account in the book of 4 Nephi.
Second, I have speculated that Ammaron buried the plates because a coup ousted his family from power and also endangered the plates. (See commentary accompanying 4 Nephi 1:22, 48–49.) If this is correct, then Mormon would have to be distant kin or, as a potential claimant to the throne, he would also have been in danger.
It is possible, though speculative, that Mormon’s father was Ammaron’s court retainer. Such a relationship would allow enough social distance between Ammaron and Mormon for Ammaron to feel that Mormon was not in danger from his own enemies.
Ammaron’s description of Mormon as “quick to observe” may mean that he learned quickly or was a bright student. In this case, Ammaron would have meant Mormon’s ability to read and write, required traits for a recordkeeper. Maya practice suggests that the literate were most likely nobles outside direct inheritance lines. This is, of course, exactly where my speculation would place Mormon.
However, “observe” might also mean obedience to orthodox religious practices. Although there is nothing implausible in such a reading, it seems more likely that Ammaron’s interest in a ten-year-old would focus on his literacy (not necessarily typical of a youth) rather than his piety (which Ammaron may have assumed based on Mormon’s family and heritage).
Chronology: The probable timing of Ammaron’s interview with Mormon would be the year Ammaron ended his record, 320 years after the Messiah’s birth (4 Ne. 1:48). Mormon, therefore, would have been born 310 (Nephite) years after the Savior’s birth, or A.D. 301.