I think you can use "their" in those cases in which you dont know the gender of someone
"Their" Is the correct term. You can also use it when you know their gender as well.
For example, if it's a fetus and you only know about the finger in its mouth via ultrasound
The grammatical gender of a child is 'it'. Please note there is not always a one to one relation between grammatical gender and biological gender. For example, large vessels are referred to as a she whike you woukd expect it. Or a mouse, insect etc is an it (even though it can be a he or a she).
Many pronouns are gendered. "It" is the neuter pronoun, and as this person describes, also used on occasion when the correct gender is unknown.
Interesting fact: In Greek, the words for "boy" and "girl" are neuter, grammatically. The more you know.
I'm no english expert, but can you use possessive "their," if talking about only one child? You'd have to say "the child has his/her finger in his/her mouth"
In recent years it has become more and more common to use "their" as a gender-neutral singular possessive pronoun.
If by "recent years" you mean a few centuries then I agree. It was common even as early as in the 16./17. century and some even claim the singular "they" was around since the 1300s (although I haven't had the time to verify it myself yet, so I don't know how valid those claims are.) Only later grammarian prescriptivists started to have a problem with singular "they" and only now it's coming back to graces. (Just like grammarians also used to have a problem with - also perfectly valid - finishing sentences with prepositions)
Yes, it's the third person singular form of "they/their" and it is normal usage.
Sorry, folks, but to this Old Fart (not to panic, those of you of tender sensibilities, the term is a very, very mildly risque euphemism for one of advanced years), the general acceptance of the singular “their” is proof positive of advancing entropy. Abandon all hope; surely, The End is near; Doomsday is rapidly approaching!!!
How else to refer to a person if it isn't known if they're a man or a woman? For example:
"My cousin from America is coming to visit me next week." "Oh really? How long will they be staying with you?"
Your example is perfectly acceptable. I faced a similar problem some time ago, writing text for the UI of a social networking website. I needed to write things like "Your friend would like to share their pictures with you - accept?". That sort of thing appeared everywhere throughout the site, and the alternative of using the clumsy "he or she" and "his or her" all over the place was a non-starter :)
An answer to this question has to start with realizing the quite odd meaning of this sentence: the child has for some reason inserted its finger into an unidentified mouth that is, implicitly, not his or her own, but, yet, the owner or nature of that mouth (is it the mouth of a volcano or a person?) is remaining consciously unidentified.
There are times when translating between languages that context has to either be inserted or removed to yield a natural result in the target language [just like the explicit possessives have to be removed translating the English version of this sentence to Russian]. Not a native Russian speaker, but I think this would be one of those cases: something about which mouth this finger has wound up in would be included. "The child has its finger in someone's mouth," or what have you.
Textbooks seem to say something like во is used "before difficult consonant clusters" and sort of leave it at that. Probably b/c the matter is a touch complicated. I found this more comprehensive listing: http://ask.masterrussian.com/279/what-is-the-rule-for-using-%D0%B2%D0%BE-%D0%BA%D0%BE-%D1%81%D0%BE-instead-of-%D0%B2-%D0%BA-%D1%81
A few short masculine nouns have a separate form for В and/or НА in the locative meaning. Such forms always end in stressed У and are, by a large margin, obligatory.
Compare to в лесу, в аэропорту, в саду, в порту, на берегу реки.
I accept that this sentence means what it means, but I can't really understand why. It seems to say only that "By/at a child there is a finger in a mouth" :) How does the finger and the mouth belong to the child?
A person's own body parts (or relatives, as another important example) are not prefaced with possessives in Russian unless for some reason there would be a lack of clarity about whose body parts are meant.
у + person in genitive is a structure that is used to indicate possession in certain cases, particularly when that possession is extremely "deeply rooted," like parts of one's body, places one lives.
Got it, thanks.
I'm firing a lingot your way (and not just because I have more than I know what to do with :)