The Last of the Really Great Whangdoodles — Julie Andrews Edwards

Ben, Tom and Lindy Potter are ordinary children. They lead ordinary lives -- until they meet the brilliant Professor Savant. He tells them all about the Whangdoodles, the wise and magical creatures who once lived on earth but then disappeared to another land, and were forgotten forever.

Except by Professor Savant and the three children, who are determined to visit Whangdoodleland, where the last of the really great Whangdoodles rules over his kingdom of fantastic creatures. Getting there isn't easy. The children must look beyond the ordinary, and see things in a completely different way. It's hard and dangerous work, and sometimes it seems that they'll never reach the Whangdoodle's castle. But little by little the children learn that with a bit of imagination, anything is possible.