Gepatroj doesn't really mean "parents" but rather "at least one mother and father". It is distinctly different from the English word "parents" because "parents" is gender neutral while "gepatroj" is dual-gender. "Parents" doesn't imply any gendering, "gepatroj" implies exactly two genders.
The English "parents" cannot really be directly translated. You must add genders and then translate as "gepatroj", "patroj" or "patrinoj".
This is an interesting question but the only response I can possibly think of is that tri is not an adjective...but that raises the question of what part of speech it is. For example, "La Unua Libro" literally translates to "The First Book" (which was written by Zamenhof) but that is when it is turned into an adjective to modify a noun.
I looked for a link and this wikipedia article might help, go to "numbers" - https://en.m.wikipedia.org/wiki/Esperanto_grammar
There it says:
" These are grammatically numerals, not nouns, and as such do not take the accusative case suffix. However, unu (and only unu) is sometimes used adjectivally or demonstratively, meaning "a certain", and in such cases it may take the plural affix -j, just as the demonstrative pronoun tiu does:
unuj homoj "certain people"; ili kuris unuj post la aliaj "they ran some after others". ..."
TL;DR - numbers are considered numerals and do not take the accusative case suffix
Could someone clarify the meaning of "havas"? I usually think of "have" as indicating legal possession of material objects, and I've translated that one meaning to "havas," but people aren't treated as material objects (in most cultures).
I think the phrase "have kids," "have daughters," or "have girls" are using a slightly different version of the English word "have." "Have kids" also has a rather different meaning--"reproduce"--instead of the strict semantic "raise/own kids." I feel like "havas" is more specifically translated to "raise" or "guard/keep" here, but I'm not sure.