In the novel to they explain why the young kid gang is trying to kill Fanucii ?
I think they were simply "teaching him a lesson," as it is referred to.
Had it not been those, Don Vito Corleone may never have been "Don," as their attack gave Vito the confidence against Fanucci.
Mick
In case it's still bothering you, here the full quote from the novel itself:
"The three young men [who attacked Fanucci] were not murderers, merely tough young boys determined to teach him a lesson and stop him from scavenging [around the neighbourhood]."