The Sindarin word for “eye”, most notable in the name Amon Hen “Hill of the Eye” (LotR/400), derived from the root √KHEN that was the basis for eye-words (PE17/187). Given the words henneth “window” (LotR/674) and Lachend “Flame-eyed” (WJ/384), it is possible that the independent word for “eye” is henn or hend, but note also maecheneb “sharp-eye” which has no double-n (WJ/337).
Conceptual Development: This word dates all the way back to G. hen “eye” in the Gnomish Lexicon of the 1910s (GL/48), cognate to ᴱQ. hend- and so probably similarly derived from primitive ᴱ✶þχe-ndǝ (PE12/21). In the Early Noldorin Grammar of the 1920s, ᴱN. hen(n) “eye” was paired with ᴱQ. sinda (PE13/122), but in Early Noldorin Word-lists from the same period, ᴱN. henn was again cognate with ᴱQ. hen (hend-), both from primitive ᴱ✶ske-ndá. In The Etymologies of the 1930s it was N. {hent, henn >>} hên “eye” from the root ᴹ√KHEN-D-E “eye” (Ety/KHEN-D-E). Thus this word was well established in Tolkien’s mind, but had several variations in its form and derivation.
A word for “child” derived from the root √KHIN, more specifically from ✶khinā with short i which became e in Sindarin due to a-affection (WJ/403). It often appeared in its mutated plural form chîn in phrases like Narn i Chîn Húrin “Tale of the Children of Húrin” (WJ/160). This is pronounced with spirantal “ch” as in German Bach, not affricate “ch” as in English “church”.
Christopher Tolkien made the editorial decision to render this plural form as Hîn in The Silmarillion as published as well as in Unfinished Tales, where it “was improperly changed by me [Christopher Tolkien] to Narn i Hîn Húrin ... because I did not want Chîn to be pronounced like Modern English chin” (LR/322). It seems Tolkien himself had similar concerns, as he sometimes rendered its Quenya cognate as sén, which would have Sindarin forms ✱sên “child” and ✱i hîn “the children”. However, Tolkien’s motive was probably a desire to retain the early (originally Adûniac) form Ad. Eruhîn “Children of God”, which in Sindarin otherwise became Eruchîn (LB/354).