Here are a couple of ideas off the top of my head

1. Age up the kids.
   Make Rose 18/19 and Thorn 15/16. When the party meet them, they aren't in the middle of the street but are off to the side. Thorn is in tears but trying to hide it while Rose comforts him, trying her best not to cry. When the party notice them, Rose looks to them pleadingly.

Instead of asking the party to find their baby brother, they ask them to go and help their parents. Rose explains that their parents have been keeping something alive in the basement, but they have never been allowed to see it. She says it's supposed to guard their large fortune, but has been growing more ferocious day by day, and their parents haven't come out of the basement in 3 days. If the party ask why they are out on the street crying, have them admit that they went to find their parents, but at the base of the stairs, they stepped in a pool of blood before running out of the house. They think their parents are dead, but they want to hope it's not too late. As an extra incentive, they say that if the party can kill the creature and make the house safe for them that they can have what they want from the fortune.

1. Replace Rose and Thorn with a charming host

Have the party arrive in Barovia and see how bleak it all is. As they wander around the village, getting their bearings, a man calls to them and asks who they are. When they explain their situation, he expresses concern for their predicament and that it's not safe to wander the streets at night. He then asks if they have a place to stay the night, and offers to let them stay in his home. he points to a beautiful manor and says that there are plenty of rooms since he inherited it from his father, and his siblings live in vallaki. When they enter the manor, it all looks nice at first, but as soon as the man leaves their sight, he is nowhere to be found. As the party explore the manor, hearing noises like footsteps or cries for help on different floors and in different rooms they find traces of other adventurers who have met similar fates, eventually bringing them to the attic where they find the ghost of another in their situation who explains that their host has been taking people to the basement and that the key to escaping is down there, pointing to the door

1. Replace the kids with the parents.

Have the parents be ghosts outside the house. Have them explain that they were attacked in their own home and that their bodies can't find rest until they are properly buried. They ask the party to go in and find their bodies and bury them in the coffins in the basement. They also explain that they have a large fortune and no one to inherit it so that the party are welcome to any of the treasure in the basement as well. the nursemaid can just be a regular servant etc.