What if their kingdom was destroyed in the ogre wars? From episode Skin Deep we know the ogre wars are still raging recently. (Recent in relation to the time when the big curse was enacted.) It was mentioned on one of the past podcasts; old Europe (in which these fairy tales are based) had many, MANY kingdoms. Each city had their own king, pretty much. So it’s not far fetched that Regina’s dad could have been another king.
As for Rumple involved in a lot of trades of children, one theme with him is trading children either for royalty (ala Prince James) or taking them. But the trade usual involves a royal.