The setting for the adventure is not important. The below write up is for a pseudo-Victorian setting, but in the past, I've DMed essentially the same story in an Ancient Egyptian household, a Roma camp and a generic fantasy city.
Note: "Think of the Children!" is a little darker than the average kid game. It includes the off-screen death of a child and might include animal cruelty. If you feel these are not appropriate, feel free to replace the death with a coma and make it clear that the villain can be exorcised without the need to destroy the host body.
Background
Our story begins with the PCs being invited by the wealthy patriarch of the Le Fanu family to investigate a mysterious condition afflicting his children. Two weeks ago, his daughter Bertha became lethargic and anemic. All forms of curative and mundane healing brought only short respites before her symptoms returned. Last week, she died. The following day, the younger daughter Laura started displaying similar syndromes.
Unless the PCs already have characters, I suggested giving each player 1-3 random divination spells for a total of eight spells for the entire party. This adventure is practically impossible to solve without detect magic. The ability to speak with animals makes it significantly easier to solve.
I use this map for the mansion (only without the collapsed walls). Because the adventure is a location-based mystery, it's impossible to predict the order and nature of encounters. How this story goes is 100% dependent on PC actions.
The household contains the following characters:
Adults
The Right Honourable Joseph Le Fanu, 7th Lord Clifford: the children’s authoritative father, a cold and imposing aristocrat whose stoic upbringing gives the impression of callousness or even cruelty.
Mina (née Arshanavat Van Helsing): the children’s grief-stricken mother, a gentle half-elf who finds it difficult to cope with the loss of her eldest child and the current condition of her youngest daughter.
Tagore: a meek and cowardly chef from the East, has a rather scary collection of exotic cookbooks (including one written for and by dragons) that he keeps for purely bibliophilic purposes.
Funny and Sunny: two mischievous maids who live in a tiny room, sleep on a bunk bed, and keep a secret pet rat named Milord. Their tricks antics are annoying and inappropriate, but ultimately harmless. If the PCs lacks magic users, you can give the two limited spellcasting ability and make Milord Funny's familiar.
Children
Bertha: dead and buried, her cat Tubby never left her side as she lay dying.
Laura: a graphomaniac diary keeper, presently drained of vitality to a condition of near catatonia. Only wakes up due to curative magic and languidly answers 1-3 questions before falling asleep again. Her cat Bandit never leaves her bed. Investigation will reveal small bites on her body, consistent with the fangs of her cat.
Max: an extraordinarily rude, spoiled and hotheaded boy with an equally belligerent parrot named Killer. Constantly plays the violin, terribly, in preparation of becoming a world renowned artist. Likes to bow before the mirror. Shouts at anyone asking him to do anything.
Jack: a nauseatingly friendly lad who works at the stables, wishes to help the PCs but has the intelligence of a chair and only shuts up in the presence of Lord Joseph. Lacks the intelligence to understand he's not wanted, not matter how meanly the PCs treat him. As long as he's with the PCs, stealth is impossible.
Pets
Tubby: Bertha’s orange cat, skulks about the second floor, obviously depressed.
Bandit: Laura’s giant Angora cat, never leaves her bed, currently possessed by Count Orlok, who uses the fat cat to drain the girl's blood.
Milord: Funny and Sunny’s pet rat, lives inside a locked box in their room. If Milord is used as a familiar, then it can help the PCs as a spy. It can also report that something is off with the household animals.
Killer: an ornery parrot that lives in an open cage in Sheriden’s room, knows lots of curse words and uses them liberally and at the most inappropriate times. If attacked, he flees to the lord's study, telling on the PCs with his limited vocabulary.
Dog: the lord’s wolfhound. Joseph is not so sentimental as to name his animals. Obligingly, the dog is also lacking in any character traits except for mindless loyalty to his master. Dog is the most powerful animal in the house and can be used as an antagonist for a final climatic battle against Count Orlok.
Villain
Count Orlok: the ghost of a vampire Mina’s father dispatched more than a century ago. While powerless by himself, he found that he can possess animals and use them to drain blood. He can jump from a body to body as a move equivalent action, provided that the animals are alive, and no more than 10 meters apart. If the animal he possesses is killed, immersed in holy water, or targeted by turn undead, he’s forced to manifest. If this happens during daylight, he’s automatically destroyed and the day is won. If manifested during the night, he must be destroyed by some other means. Anything affecting a normal vampire also affects Count Orlok. One group defeated him by spilling rice (which he was obliged to count) until sunrise. I thought it was pretty clever.
It takes Count Orlok a week to fully drain a child. While in animal form, he can use sleep and charm person as gaze attacks, or by licking his victim. He can also use any natural attack the animal has. However, the Count uses violence only as a last resort as he knows how vulnerable a small animal’s body is. Instead, he jumps from body to body, tries to frame the servants and, if all seems lost, tries to flee the mansion in the body of some stray animal.
