https://laracasts.com/series/build-mobile-apps-with-react-native-and-expo - I see you are not subscribed, but Andre there builds an app using react native, and here are the repos
https://github.com/laracasts/lc-rn-twitter-clone -> ReactNative App
https://github.com/laracasts/lc-laravel-twitter-clone -> Laravel Backend
And here's how authentication is handled: https://github.com/laracasts/lc-laravel-twitter-clone/blob/master/app/Http/Controllers/AuthController.php#L38
hope it helps a bit, you may send a XML response with the token, instead of JSON.