Beautiful cover with a picture of an opening through a Forrest. The colours were browns and golds and green. It was a story about a boy and girl traveling into an enchanted place and the things they experienced there, an adventure. I read this book over 50 years ago
I don’t know if this is a picture book or novel but it could be Bridge to Terabithia by Katherine Paterson?
Perhaps Mary Stewart’s A Walk in Wolf Wood?
What about “The Music Man?” I don’t know the author, sorry. But the book involved a forest and two children on an adventure. I think there was also a raft and a victrola involved.
It sounds to me like it might be The Magician’s Nephew, one of the books in the Chronicles of Narnia. The cover on my sister’s books showed Digory and Polly in the Wood between the Worlds.
If a Narnia book, I would think it would be The Silver Chair. The frontispiece picture can be found here: https://www.thefandomentals.com/silver-chair-chronicles-narnia-reread-project-part-6/
If you didn’t specify 50+ years ago, I would suggest The Beginning Place by Ursula Le Guin.