Since your daughter said, "(Somebody) is swimming." it is grammatically correct to say, "Who is swimming?" in response since the phrasing makes it seem like only one person (singular) is involved. If she had said, "Some (people) are swimming." (plural) then that would be the right time to ask, "Who are swimming?".

In American English, "who is swimming" is standard. I believe this might differ by country though; one of the differences between British and American English is the choice of verb conjugation in certain situations, and I'm not entirely sure where all the differences come up. For example, American: "Boston *is* playing New York (in a baseball game)," but British: "Tottenham *are* playing Arsenal (in a football match)."

Oｆ　ｃｏｕｒｓｅ　ｕ　ｃｏｕｌｄ　ａｓｋ　ｉｎ　ｔｈａｔ　ｗａｙ　ｕ　ｔｒｙｙｉｎｇ　ｔｏ　ｄｏ． - Somebody is swimming. - Who is swimming? - A man or a woman. My daughter and　　Ｉ 　　were near　ｔｏ　 a swimming pool and there were two people in the water. My daughter said: someboｄies 　ａｒｅ　swimｍing. But there were two people in the pool! Should I ask her "who are swimming?" in this situation? Thank you for your help!