"I have five children."
Translation:У меня пять детей.
Not never, but it would be rather awkward in most situations describing kids. Maybe if you were describing someone else's children, preferrably in a very pedantic way, naming each of them and their feats, too...
However, it is not as bad as with body parts. Just contextually very limited because people usually say the number of their children in a pretty predictable set of contexts (like, talking about their life and their family). Though, with objects it is also not that common, as long as you say how many you have.
They are both possible. Russian has so-called collective nouns двое, трое, четверо, пятеро, шестеро, семеро. Very rarely, десятеро (which to my ear is already thrice as funny as saying "thrice" in informal English). Others are virtually never used. Collective nouns can be used (and are used) with masculine and common gender nouns that mean people, as well as люди, ребята, дети, лица (a formal way to say "person"). Use the Genitive plural of a noun:
- двое детей, двое друзей
- трое рабочих
- четверо пьяниц (4 drunkards)
- пятеро ребят
Using a regular cardinal number is also fine. This course does not teach Russian collective numerals: they do not translate to English anyhow, so the exercises with them would be extremely frustrating.
Some speakers occasionally use these special numerals with nouns for feminine persons, at least is spoken speech. It is frowned upon, and I do not know what will come sooner—acceptance of such use or a major decline in popularity of the collective numerals. Given that in modern language the connection between двое, трое and the gender of the noun is obscured, I think it would only be fair if these became unmarked for gender in future. Right now it is okay-ish in informal speech, sometimes, but is considered poor use for written speech.
Most importantly, collective numerals are used with plural-only nouns. You might recall that 2,3, and 4 require Genitive SINGULAR. This requirement becomes very, very problematic for nouns like "scissors" or "pants" that do not have any singular form—so a collective numeral is put there istead.. Saying someting like "22 scissors" is still a problem, though. You might use "штука" (item) to get aroung this problem... Or you can just never talk yourself into a corner. After all, you rarely HAVE to say large and precise numbers. Languages let you say "37462 men", "23 eyeglasses" or "50064 pants" but in general people get away with approximate amount just fine: "over 37 000 men", "about two dozen glasses", "50 000 pants" are OK for practical purposes. I think this is why the numbers system in Russian turned out that bizarre: when it was still in the making, it is likely that most people rarely needed large AND precise numbers in speech in combination with nouns.
Now about stuff we do teach (a little bity). Russian also has words for "together" that are formed from numbers and mean the company of N people (as compared to вместе "together", which does not specify how many people are together).
- вдвоём and втроём are the most popular
- вчетвером, впятером, вшестером, всемером and вдесятером are also used sometimes
- ввосьмером, вдевятером and words "together" for companies larger than 10 people are barely ever used—I do not think I ever heard it IRL.
The effective plural of ребёнок is дети. Ребята is said about guys/boys, and also (sometimes) as a form of address to a group of kids (with boys and girls).
Is not дети a plural-only noun? So why is it declined in feminine? Is there any rule about this?
Strictly speaking, дети is originally the plural of дитя, which is a plural noun. The Genitive plural is indeed детей.