Well my heart did a little leap of joy when I read that first part about successfully ordering and receiving books from a Norwegian bookseller. Were they physical books?
I am sure you will get better answers than mine, and I look forward to other responses. I
love children's lit.
Since you have the Moomins on your list, I presume you are open to books written in Swedish. If so, I would add Astrid Lindgren. My favorite of hers is Ronja the Robber's Daughter. (Digression: I have read a lot of both Tove Jansson and Astrid Lindgren in Danish. I love their stories so much and hope to try them in Swedish someday.) And what about H.C. Andersen from Denmark?
The following are Norwegian authors. I have either read these (in Danish) or I want to read them.
Karius og Baktus by Thorbjørn Egner. For kids. Originally published in 1949, so I guess that makes it a classic. Is it still popular in Norway? I don't know. Maybe someone will tell us!
Snøsøsteren (2018) by Maja Lunde. For kids. Christmas theme.
Sophies Verden by Jostein Gaarder. YA. Kind of a unique book that is a tour of philosophy.
Folkeeventyr by Asbjørnsen and Moe. Classic folk and fairy tales.
For Folkeeventyr, here is a website:
https://folkeeventyr.no/Ravneringene by Siri Pettersen. YA Fantasy trilogy.