That doesn't sound like a nightmare, that sounds like what they call Night Terrors. My son had them when he was 4 for about a year and it was horrible, but he seemed like he was awake and hallucinating but he was actually sound asleep. Maybe your dcfamily should check into that? My son was 4 in the 1980's so they may have a whole new explanation or name for it by now but it's worth checking out.