"How old is your child?"
Strictly speaking, this is grammatically correct. However, it seems unlikely that you would ever use this in actual conversation.
(1) You would rarely refer to another person's child as こども directly to that person's face. Instead, you would probably say お子さん (おこさん).
(2) You would probably not be using あなたの. If you have gotten to the point at which you are discussing someone's children, you will probably know that person's name. So, instead of あなたの, will use that name, i.e., 〇〇さんの.
(3) However, even then, there is a chance you will not use the name, If you are directly addressing someone, and it is clear that it is that person's child you are talking about, you won't say 〇〇さんのおこさんは, but rather simply, おこさんは. (Note that, if you are speaking in a group, to more than one person, and you need to distinguish one person from the others, then you would say 〇〇さんのおこさんは ...)
(4) If it is a friend or an acquaintance, you might ask なんさいですか. But if it is someone who is not a friend or acquaintance, it is better to ask おいくつですか.
So, in sum, it is probably better to translate this as おこさんはおいくつですか。
2020.4.24 The Japanese are often very forgiving with beginning learners with the formality of their language. They know you aren't a native speaker who grew up in Japan. So as long as you're understood and don't use too crude of a language style (for example your typical action anime), everything will be fine.
お子さんはおいくつですか would be probably the way the Japanese would ask to a Japanese stranger, or an intermediate\advanced student of Japanese, but if you just said
こどもは いくつ or こどもは いくつ ですか
most Japanese would just be happy everyone is communicating
you could use かれ and かのじょ but those pronouns are commonly used to mean “boyfriend” and “girlfriend” irl, so it kind of depends on the situation.
if you know the person’s name, use their name; or just leave the pronoun out altogether - like 「お子どもは何さいですか？」 - if it’s obvious who you are referring to.
if not, 「あなたの…」or 「あの人」(あのひと, “that person”) should be ok, but maybe not used repeatedly in the conversation, as well as 「彼」 and 「彼女」 as long as it won’t be mistaken for bf/gf.
Is there another way to say ‘he’ or ‘she’ without possibly confusing it with girlfriend or boyfriend? Or something… I get that we should use names more often, instead of things like ‘I’, ‘me’, or ‘you’. However, sometimes that can get… redundant. For example – Imagine two women bumping into each other that haven’t seen each other in years. Woman 1 wants to ask about Woman 2’s child’s age, then goes on to comment about how he has his father’s eyes.
Dialogue: Woman 1: It’s been awhile! How have you been? Woman 2: Good, thank you. How’re you? Woman 1: Good! Is that your’s and Andrew’s son? Woman 2: Yes! Woman 1: How old is he? Woman 2: He’s four years old. Woman 1: Oh, wow! He has his father’s eyes!
So, see? Something like this you would have to say the kid’s name 3 times (at least), if you don’t say he somewhere in there, it would be awkward, wouldn’t it? At least from an American point of view anyway… Skipping to the last three lines, it would be something like this, right:
Woman 1：彼は何歳ですか？ (kare wa nansai desu ka?) Woman 2：彼は4歳です。 (kare wa 4sai desu.) Woman 1：ああ、すごい！彼は父の目をしている！ (aa. sugoi! kare wa chichi no me oshite iru!)
Or are there better options? (Sorry for the random dialogue and I hope my Japanese isn’t too crazy lol)
haha, good example! yes, not including the pronouns can get confusing, although even in that example in english, Woman 1: How old is he?, the "he" could refer to either the son or Andrew, depending on how one reads it.
i'm just guessing, but after the first 「彼」 in that exchange, the "he" and "him" pronouns would be omitted and Woman 1 would either:
ask what the son's name is so that she could refer to him accordingly (and she already knows the husband's name is Andrew);
use hand gestures so Woman 2 knows who is being referred to without the pronoun being included; or
refer to the son by his "polite pronoun title", maybe 「お子ども」or 「お息子」 (o-musuko?)?
makes me wonder how many japanese conversations end up sounding like "who's on first" haha.
2020.4.30 I'm just gonna translate your whole dialog into spoken Japanese, so you can see how the context and conversation would flow. Also, I'm gonna be dropping particles and keeping it informal, since it appears to be a dialog between friends
So you see I haven't used 彼 nor 彼女. I used the name Andrew once and use 子 to refer to the kid once. Later Andrew was お父さん. The subjects and objects introduced once in the conversation are assumed to continue, until another new ones are introduced.
English has its distinct way of communicating. Languages that don't say the subject & object often (such as Japanese) has its own distinct way of communicating.
Also about she/girlfriend & he/boyfriend, that's taken care of a lot through the same way. If there could be confusion, the listener usually will ask for clarification, like
彼って言いましたが彼氏のことですか？ You say kare, is that kare as in boyfriend?
Let's say there is another scenario where 1) and 2) are dudes at a bar. There's a random girl drinking and 1)'s gf is buying a beer next to her.
2) １）さん 彼女はいますか？
[1)'s name] , do you have a girlfriend?
1) Yes, I do. Look towards the bar. She's buying a beer now.
２）なるほど。となりの彼女は？ or となりの女の人は？
2) I see. How about the girl next to her?
1) I don't know. But she is cute.
The Japanese by far will ask more 誰が 何を 何が どこに type questions to clarify the context, than English speakers that explicitly say almost everything, every sentence.
No, children are not referred to as sama by an adult. Maybe the children's junior peers might say something like that if they were in an anime, but probably would say anything more honorific than senpai. No matter what the case is, you would not refered to someone else's kids as -sama