When I was a kid in the 80s, I had a paperback copy of a children's novel in which a brother and sister (orphans, I think), went on a journey and met what I think was a talking griffin, who guided them through the rest of their journey. I think the antagonist was an evil wizard or something like that. The clearest identifying detail I can remember about the story was that at some point they passed through a cave and ate cave moss, which was described as having a surprisingly pleasant, tart flavor. At the end, they were either adopted by a king and queen, or it was discovered that they were the king's and queen's long-lost children.
I have no recollection of receiving this book, so I think it was probably a hand-me-down, likely published in the 70s or even 60s. The cover of my edition had a red-on-green color scheme, with (I think) a picture of the griffin. IIRC it was more or less human-shaped with a parrot-like head, but I'm really reaching here. There were several beautiful, almost photographic, black-and-white illustrations scattered throughout the book (maybe at the beginning of each chapter?). IIRC it was 100-200 pages.
The book must be fairly obscure, and is likely out of print, as I searched through a list of hundreds of children's fantasy books and turned up nothing.
Edit: It's not this.