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).