children's picture book about siblings finding a portal (a mirror?) to their cousins from around the globe from 90s or 60s

[Hannah]

This is a book from my childhood in the the 90s, though it could be from my parent's childhood of the 60s.
Plot: Two children find a magic mirror (box? pool? otherwise reflective surface) and they can see/communicate with their "cousins" from around the globe - including eskimos and japanese children. I think eventually they decide to cut off communication.
Beautifully illustrated in colorful and bold style.

I thought it was called "the magic mirror" but I cannot locate it under that name.

[Lobstergirl, au gratin]

Hannah, are you still looking for this or did you find it?

[Capn]

There's a list for that, if you are still here....
https://www.goodreads.com/list/show/3...