Can you suggest a book about a child journeying to find a lost person

Hello,

I would like to read a children's book about a child journeying to find a lost person. That person can be a parent (for example, a father who moves away after a divorce) or anyone else (for example, a friend who moved away). The journey itself can be in the real world or have elements of fantasy or sci-fi.

Would appreciate any suggestions.

Thank you in advance.