Found: Unknown Title and Author

I've been trying to find a book that I remember very little about except the book's blue dust jacket had a door cutout in the center of it to an image plastered to the fabric texture of the front cover to what may have been Atlantis
Very adventure fantasy esque book
I think there had been a girl or maybe a boy and a girl with a key to said door. I'd found it at a book shop sale nearly 20 years ago and have since lost it and would love a chance to have another copy

Prospero's Children? The colors aren't quite right but there's a door cutout and Atlantis.

>2 keachachu: YES! That's the one! You're an absolute genius! Thank you so much! I must have only been remembering the blue in the window