What's the Name of That Book??? discussion

Hi guys,

I would like to seek your help looking for story book I read during my childhood, I couldn't remember the title nor the author.

There are two main characters in the book, a boy and a little monster (I couldn't remember the names as well). The boy likes eating junk food while the little monster likes healthy food (and both of their parents like the opposite of what their kid likes). One day, by coincidence, the boy realized that the little monster lives right underneath their house. After a conversation, they decided to swap families so they could enjoy the food they like.

However, they each had too much junk food and healthy food respectively and felt sick. So they decided to return to their own family and have they food they were supposed to have.

I hope that's enough information for looking for that book, thank you very much in advance!

Oscar, please put genre and plot details into your header/title. This could help get your thread solved more quickly.