Shinseki No Ko To O Tomori =link=

The phrase "" (親戚の子とお泊まり) generally refers to a scenario involving " staying overnight with a relative's child ."

If you are writing a "slice-of-life" story about a person babysitting or hosting a younger relative for the first time: : shinseki no ko to o tomori

“O-Tomori” might be a surname or place name. For example, a village called Tomori. Then the phrase means “The relative’s child and O-Tomori (a person or place).” shinseki no ko to o tomori