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).
In the Quendi and Eldar (Q&E) essay of 1959-60, Tolkien said “S has hên, pl. hîn, mostly used as a prefix in patronymics or metronymics” (WJ/403), meaning this word was often used to mean “child of” in reference to one’s parents. For example: Túrin hên Húrin “Túrin Húrin-child” or Túrin hên Morwen “Túrin Morwen-child”. These are functionally juxtaposition genitives without mutation of the following name, as in the example given by Tolkien: Hîn Húrin “Children of Húrin”.
The most common Sindarin word for “daughter” (SD/129; VT50/18).
Conceptual Development: In the Gnomish Lexicon of the 1910s, the word for “daughter” was G. suil with shorter form sui (GL/36, 68) probably derived from the early root ᴱ√SUẈU (QL/87) and replacing rejected {thuai, thuil} (GL/36, 73). In The Etymologies of the 1930s Tolkien gave (archaic) N. †sell “girl, maid” derived from the root ᴹ√SEL-D (Ety/SEL-D). Tolkien said sell was replaced by N. iell “daughter”:
> with i from iondo son [YŌ]; a change assisted by the loss of s in cpds. and patronymics: cf. Tinnúviel < ✱tindōmiselde (Ety/SEL-D).
Thus in Noldorin, sell became iell under the influence of N. ionn “son”, and assisted by the fact that intervocalic *s* became *h* and then vanished when -sell was used as a suffix in compounds.
It seems Tolkien abandoned this 1930s paradigm, however, since he used sell for “daughter” in a couple later documents, namely the King’s Letter (SD/129) and the Túrin Wrapper (VT50/5).
Neo-Sindarin: For purposes of Neo-Sindarin, I would use sell for “daughter” but suffixal -iel for “daughter of” under the influence of -ion “son of”, a scenario similar to that of The Etymologies. I would also allow iell as a less commmon variant for “daughter”, derived from the suffix.