For advice about calming a child after having vivid dreams (from someone who always had them as a child, and it was definitely not an option to get in bed with parents or have mom come into bed), I would say drinking water absolutely helps (if you haven't tried that) and rubbing their back is a really good idea. And just firm reassurance that it was not real and they are safe in their beds. A flashlight, nightlight or one of those things that puts the stars on the ceilings is a good distraction after you've left them. Sitting up for a few minutes in bed helps get that distance between the memory of the dream and being back to reality. But if you don't want it to turn into some manipulative thing, being constructive (water, assurance) but firm and leaving them to it is probably best.
When I was older my sister simply hurled a pillow at my head and that got me back to reality pretty fast. Of course a sister can get away with that...
