Neuter term for "child"
What would the Romans say if the gender of a child is unknown?
Acc. to Smith and Hall's English-Latin dictionary, puer, pueri. Children might be referred to as liberi, but only in relation to their parents, rather than in general. Also Lewis and Short's Latin-English dictionary says the same, as does the Oxford Latin Dictionary ("puer" def. 4, "liberi," leading def.). And that's what I've seen when reading (but I'm no expert).
The two dictionaries linked to you'll find very useful for being "more of an expert," even if they are 150+ years old. The other (OLD) is relatively quite new but not online . . . and FWIW I usually refer to the other two anyway, unless a tie-breaker is needed.
. . . note, BTW, that there is another word spelled like "liber, liberi"--the word for book (originally, for tree bark). It has a short "i" (liber), whereas the word meaning a child has a long one (līber), and they decline differently. When long vowels aren't marked, context will show you the difference.
Liber, libri (book) vs. Liber, liberi (child). That should be a pretty good indication right there; the stem is spelled differently. ;)
That said, there is an adjective (liber, libera, liberum) that might pose a greater issue ... it means free, independent, etc. (See full definition here: http://archives.nd.edu/cgi-bin/wordz.pl?keyword=liberi) This one you'll probably have to rely more on context to tell you the difference between them.
Have fun with Latin! Best of luck! :D
Volgav vitsenanieff nivya kevach varatsach.
If the gender is unknown (or there is a group consisting of people of various gender) they often use the masculine term. Speaking of children, the most neutral words might be 'liberi' (attention: plural only, not to confuse with liber, libri m. - the book) or 'infans' (which can be masculine or feminine). And then there are some poetic words like 'proles' (the offspring) and other words that demonstrate the family relation ('semen' the seed = the child; 'sanguis' - the blood, but also the one you share your blood with = your family/children; partus, -us' - the act and the product of giving birth)
Whatever term is used for a child (puer can be used of daughters: there are some religious cult citations, in the OLD, showing, for example, that Proserpina is called the puer of Ceres; there's also both puera, for girl, and puellus, for a boy--apparently, with sexual connotations), no term for a person, in Latin, is "neuter."
Neuter is a gender category used (only) for some non-living things. Of course, there are also non-living things that have been assigned masculine and feminine gender (based on the way the nouns are formed, not from some deep meaning).
Well--Catullus uses a word for a prostitute (scortum, -i, n.; and diminutive scortillum, -i, n.) that's neuter! But, in general, nouns for people are masc. or fem., and if it's a generic word (person, homo , for example), it uses the generalizing masculine gender.