"Who believes in the children?"
Translation:Chi crede nei ragazzi?
No, chi is always grammatically singular (unless the verb is essere); "Who of you believes in children?" would be "Chi di voi crede nei ragazzi?". If the question were "How many of you believe in children?", then it would indeed be "Quanti di voi credono nei ragazzi?".
The two words "in i" must be combined into nei: see http://italian.about.com/library/fare/blfare153a.htm
It's the difference between "i" and "gli" (ne- is the form of "in" in these compounds); handy table: http://italian.about.com/library/fare/blfare153a.htm