SOLVED. Children's mystery series set in 1950s and 21st century main character named Art grandfather and grandson characters a watch and a diary maybe involved in some way *Spoilers* [s]

[T]

The series has two stories running in parallel with the grandfather in the 50s and the grandson in the 21st century (or maybe the 90s), so they're both the same age. In the 50s, the boy runs a secret detective agency with a group of his friends but the adults don't know he's a kid.
In one of the books, one of the characters time-travels into the other character's time. I think it's the 50s kid comes to the 21st century.
I think maybe both characters are called Art/Arthur but I'm not sure.
I read this in the mid 2000s.

[Kris]

This appears to be too recent, but I'll mention it just in case - The Magic Box by L.M. Falcone? The Ghost and Max Monroe series - https://www.goodreads.com/series/1562...

Samantha, is boy's grandfather alive in the present day?

[T]

I don't think it's this but thank you. I'm pretty sure the book is British and set in London.

[Tab]

Invisible Detective: Double Life ? It's from the "Invisible Detective" series by Justin Richards

https://en.wikipedia.org/wiki/The_Inv...

[T]

Yes! that's it thank you so much