Children's book from the 60s/70s - aliens, black girl protagonist, pencil illustrations

I'm trying to remember the name of a classic children's science fiction novel. The protagonist is a black girl in the foster system whose name begins with N - Naima or Naella or something like that. She has gone through a succession of unpleasant foster homes and isn't really attached to anyone. She finds aliens (who I believe are posing as mice?) and helps them accomplish something. I think the book itself had very detailed black-and-white pencil illustrations. The book ends with the heroine deciding to go with the aliens into space. Please help me! Thank you :)