We can presume that the parents became Heartless after the events at DI transpired. I don't think that they would have had nobodies, b/c Nobodies are supposed to come from those with especially strong souls, and I don't think insignificant parents would fit the description.
But it is concerning that Sora, Kairi, and Riku have no concern for their family members at all. I mean, we know that they have them, and after Namine restores the memories, then you think they would be concerned as to the whereabouts of their family. (But how retarded would the game be if they had the parents in an active role in the game.....
Sora: Hey, can I go with Riku and Donald and Goofy to fight some dangerous enemies in a realm where light and darkness meet in the twilight of my destiny and that of the worlds'?
Parents: Not until you finish your vegetables!!)