Any kind of red cocktail will do the trick!
For adults, red Martini or dark fortified wine such as port will have the desired effect (and if all else fails, you can always shake up a Bloody Mary!).
For kids, try making drinks with blackcurrant juice (almost black), pomegranate juice (blood red) or grape juice, served with bat-shaped ice cubes.
To make your drink really look the part, mix different fruit juices in a see-through jug and just before you're about to serve it, pop in a dissolvable pastille (e.g. Vitamin C) to make it fizz and put the frighteners up your guests!