That’s a lovely idea, don’t let the long list of points scare you, your translation is already quite good!
máyá, cuimacira, néna, ruiné, olbá, roá, miué, yacincé
The accusative case only existed in an ancient version of Quenya called Parmaquesta, later the nominative/common was used instead. Since your verb forms on the other hand don’t look archaic at all, I’d suggest to drop the distinct accusatives.
nahtane
I would prefer it’s attested half-strong past nacante (√NAK-T-a > nahta-, past √NAK(A)-T-ne > nacante)
máyá
Later the spelling maiya/maia is more usual.
fíriëo
One can break up this three vowel sequence as fíriéno (cf. márie → máriéno), but your form should be fine as well (cf. sindie → sindieo), though a bit more awkward to pronounce.
mondo
I would prefer the form mundo, Tolkien used both but the latter is more in line with the Sindarin cognate mund.
yulne
-ln- is not an allowed sequence in Quenya, it changes to -ld- yielding past forms like tul- → tulde, yul- → yulde, but there is a competing form in analogy to the perfect tul- → túle, yul- → yúle. I would suggest you use the same paradigm for both verbs in a single text.
luhtyane
This neologism is somewhat uncertain and the -tya ending doesn’t make it easy to figure out a past tense form. One can make a case that the Sindarin verb luithia- “quench” is instead cognate with the attested Quenya verb luita- “to flood, inundate, drench”, which would have either a strong past luine/lúye or a half-strong form luyunte.
palpane
Due to the form of it’s root √PALAP, I would suggest that this is a TALAT-stem verb, which would result in a past form √PALAP-ne > palampe
i yacince, i homancane atya
This reads “the baby goat, that bought my father”, you want i yacince, i atya homancane to keep “my father” as the subject.
telpenten
I don’t think you can say “two silver” in Quenya because it’s a mass noun, just like you can’t say “two oil” in English. Unfortunately Tolkien didn’t leave us with much trade related vocabulary, but we know of a Gondorian silver coin, the canath “quater”, which would be casta in Quenya, so perhaps castanten.
er yacince
I’d use min instead of er, becaues it isn’t “the one baby goat, the single or most outstanding baby goat in exisance”, but merely one in its plain numeric sense; the father could have decided to buy two instead and the semantics of the poem wouldn’t massively change.
i X i Y
I think in this long chain of rellative clauses it is permissable in Quenya to drop the first i, so e. g. nacante cumacir, i nacante mondo, i yulde nén instead of nacante i cuimacir, i nacante i mondo, i yulde i nén, because the following relative clause helps to define the noun in question.
