Set in almost prehistoric times in another world, two children are kidnapped. They're seers but keep it a secret. At some point, they find missing legendary shards of a broken magic stone. They're also rescued by winged red horses.
I read it about 6th grade, so that would have been about 1982-84. I do not think it was a new book. The children were a boy and a girl, both having the gift of foresight. The only things that I remember are the shards of a green stone - they find one in each of the books. They were kidnapped with other children and they find one shard in a tunnel on their way to where the kidnappers take them. They are given a drink called Magogweird and then red flying horses rescue them. The book jacket has a planet with two moons, a red pegasus and the two children on it. It was near Madeline l'Engle's books in the Dewey Decimal system in the Middle School library.