In KH2, it's clear that no one connected to Sora could remember him until his own memories were restored, so his parents would've been a non-issue for the most part. It's possible Kairi explained the situation to them by the time they would've remembered him, but it really doesn't matter all that much, since he returns home by the end of the game anyway. And I think it's Selphie that handwaves Riku's absence, indicating that neither she or Kairi could remember how or why he disappeared (this was before Sora's memories and memories connected to him were restored). It's as good a guess as any to say that his parents probably suspected he ran away, given his wanderlust.
But yeah, this series really doesn't care all that much about parents, or blood ties in general. I'm sure it's occurred to the writers that their treatment of that particular aspect is flimsy or unrealistic, and I doubt that they care. It simply isn't the story they want to tell.