I read this book sometime back in 2012-2014.
The plot started when a family of three moved into an old mansion. They were a loving family despite lacking financial success. Soon though, the girl notices her parents suddenly becoming successful, but also detached and emotionless.
One night, she follows them only to discover that they've been going to an old tree every night and disappearing into a large hole at the base of the tree. (They either left the tree holding balloons or went to the tree holding balloons.)
When she follows them in, she witnesses her parents souls being put into balloons and the light leaving their eyes just a little more. The demon (?) in charge of this tried to convince her to join her parents but she runs away. I'm pretty sure she met a boy next door who joined her and he was convinced and ended up selling his soul too.
All I can remember clearly is that their souls were trapped in balloons and the protagonist was a girl with a sort of cowardly male friend who helps. I also think she had two pets, being a cat and a dog.
I'm sorry, that's all I can remember. I've spent days searching for this book.