In a lot of the fanfiction I saw before I became a CCS fan, I remember thinking, "Jeez, all of these stories involve Syaoran being a playboy of some kind...I wonder if that's the way he really is?" I came into reading it thinking that he was like that, then got surprised again when he was a somewhat cold but very sweet young boy.
And as to the reality of their relationship...what's realistic about a girl who can control magical cards and is the strongest magician on the Earth, all at the age of 10. Isn't it just as likely to fall in love at that age? I mean, look at Chiharu and Yamazaki. They've been together since kindergarten and they are still in love.
Puberty and conflicting emotions and feelings are going to be a problem for them, but aren't they problems for all teenage couples? All relationships go through problems, and just because they start off young doesn't mean that they can't get through it. I mean, they got through all of the CCS problems just fine. I think a few fights and different opinions are gonna be a piece of cake for those two.