Hm, you make a fine point, but "chi" is always treated as singular regardless of what it implies; a plural verb only follows "chi" if it stands for an object, e.g. "chi vanno a trovare di più i figli, il padre o la madre?".

I'd' say that when the subject is not clear, Italian tend to use the singular form.

Incidentally, the use of the plural in this very specific sentence, would change the meaning completely: "Chi visitano di più i genitori, i figli o le figlie?" = "Who do the parents visit more, the sons or the daughters?"