This sentence might convey if you'd put your belief in a group of boys... For example, if you'd bet your money on a school's underdog football team winning, you'd say "who would believe in them winning?"... Bit of a stretch but I can see "Chi crede nei ragazzi?" being a viable sentence...
From the WordReference definition of credere and the examples given ( http://www.wordreference.com/definizione/credere ), I think "credere in" conveys the meaning of belief in the existence of something (as in I believe in God) and having faith in something, as in the example darshanm thought of.
The meaning of belief in what somebody is telling you, or trust (I believe him), or of considering the possibility of something happening is conveyed by "credere a".
I can't be sure of this, though.
I can see you're also studying spanish... by now you should already know that in most romance languages is necessary to use the preposition for the sentence to be correct... in spanish you can remove "en" but you'll have to replace it with an "a"... I still don't know what applies to italian.
Duolingo, I can't believe this sentence is useful. Believe in God, believe in ghosts, believe in love, sure, but "believing in the children/boys" is confusing. Mods, can you change it to "Who believes the children?" This would be a much more probable and less confusing sentence IMHO.