I haven't sat and thought about this, but here are a few names-

The bloke that plays Chris in the Sopranos (for those who dont watch it, its the kid who Tommy (Peschi) shoots in the foot and later kills in Goodfellas). He could play Sonny but would he be to different looks-wise to Caan?

Also liked the Giovanni Ribisi and Gandolfini suggestions.

The movie, as a prequel, would be difficult to write and shoot I think, mainly because of the interchanging periods in GF2 with De Niro moving to America and raising a family.

De Niro would have to figure in the film somehow.

The only child-actor who I actually know the name of is that Haley Osmond (whatever!) from Sixth Sense.

How about Jude Law? He is a versatile actor but I'm not sure about the accent!

Michael Madsen was a good shout too.

Johnny Depp, perhaps? Ray Liotta?