No-Bot: The Robot with No Bottom. by Sue Hendra

Bernard the Robot loses his bottom on the park swing, and sets off to find it. Every time he gets close, it disappears again! Bird was using it as a nest, but it was too heavy; Bear used it in his drum kit, but it was too tinny; the Squirrels built sandcastles with it...and now it looks as if they're sailing away in it. Will Bernard EVER get his bottom back?