An adjective appearing as N. flaew “sickly, sick, ill” in The Etymologies of the 1930s derived from ON. thlaiwa < ᴹ✶slaiwā under the root ᴹ√SLIW “sickly” (Ety/SLIW). The ancient initial sl became thl which was the usual sound change in (Old) Noldorin, and then this thl became fl, a less common Noldorin sound change. Tolkien wrote an intermediate form thlaew between ON. thlaiwa and N. flaew, but then revised this form to thloew, reflecting some uncertainty on the phonetic development of ai in Noldorin. There was also a deleted variant of this root: ᴹ√LIW, where Tolkien had lhaew “ill” with the usual unvoicing of initial l in Noldorin (EtyAC/LIW).
Neo-Sindarin: In Sindarin of the 1950s and 60s, initial sl became lh instead of thl, so most Neo-Sindarin writers adapt this word as ᴺS. lhaew “sickly, sick, ill”, as suggested by HSD (HSD).
A noun appearing as N. lhaes “babe” in The Etymologies of the 1930s derived from the root ᴹ√LAP (Ety/KEM).
Neo-Sindarin: This word is sometimes adapted to Neo-Sindarin as ᴺS. laes, for example in HSD (HSD). This updated form is the usual revision of the voiceless initial lh of Noldorin to voiced l in Sindarin. However, this is not the only phonological difficulty in this word. For example, there is some evidence that in Sindarin, the vocalization of spirantalized p is to u rather than i, suggesting ᴺS. ✱laus. There is also evidence that voiceless stops didn’t vocalize before s in Sindarin, but rather became simple spirants, as in ach “neck” < aks, suggesting instead ✱laph. In this case, I think it is best to avoid all these complexities and just use the 1968 word S. gwinig for “baby”.