I don't remember the title so I can't find it, but what happens is there's an alien family on the run from the space-mob (or something like that) and they go into the forbidden sector to hide and end up crashed on Earth during the middle ages. If I remember correctly, the ship gets partially fixed and the parents leave without their son and daughter, who then need to fit themselves into the primitive society without the benefit of knowing the language. I can't remember if the aliens looked human or if both were something else, but the aliens definitely did not have a lot to distinguish them from the humans.