Nowhere on Earth: Galax-Arena by Gillian Rubenstein. (Simon and Schuster: New York, 1992).
Three children are kidnapped to become gymnasts in an alien circus, but the middle girl is no good and is sent instead to be a pet. There she discovers that they are really still on earth and the "aliens" are very elderly and rich humans.
The book is interesting less for the plot than for the really excellent contextualisation. The children are all from war zones, lost and unregarded. The circus brutalises them but one of the older children has attempted to create a lingua franca patois and to create some kind of dignity and collaboration. Rubenstein does an excellent job of depicting slavery, the process of divide and rule, and active reistance.