Believe in it? heh. I think it's good to think about what the likely nicks would be, and have one you know you'll like using. But I also think you can't really plan them -- people will make them up as they please, subject to the named's approval.
Lena for Illeana: not quite natural. Either only you would say it, or she'd become so used to it that she'd introduce herself as
Lena. I like
Lena.
Josie for
Jocelyn: nice, natural nick; would likely be used and liked.
Anton: Pronounced An-TONE, it kind of needs an E on the end imo. It's a natural nick though; would almost certainly stick. I like it
Bren: probably also inevitable, but only in informal situations (or by his mom). I don't feel like
Brendan really needs a nickname, necessarily.
Dominic: I like the nickname Nica, but I strongly doubt it'd stick; even if you picked it out and started using it, you might not continue unless it "just fit" the boy.
Dom seems pretty much inevitable unless you deliberately put Nica on paperwork when enrolling him in school, and never referred to him as
Dominic. I think
Nico or
Nic is a bit more likely to stick (folks resist boy names ending in A, in my experience) and to be liked by a young boy.
- chazda