Agreed. Whatever their current location, I suspect that Robb's remains were not given a "proper" burial (for the Starks, that means being laid to rest in the crypt at Winterfell with a stone carving and a sword). Leaving these remains in an incomplete and/or un-entombed state facilitates a rebirth.  I believe that GRRM uses a variation on the Celtic view of rebirth, where a person can be reborn in a baby within his own family and preferably a baby with the same name. Rebirths in ASOIAF are