We don't doubt that it happened, we're just wondering how it happened. From a coach's perspective, I've removed mini aged kids from practices before, and it was NEVER because they weren't grasping skills quickly enough. Just to provide an example, I once removed a 5 year old from my mini practice because she had gotten herself so frustrated that she resorted to crossing her arms and refusing to do anything. No amount of coaxing or reprimanding was going to fix it, so I walked her out to her mom until she was ready to rejoin us. Could someone have translated this to kicking the kid out of practice? Probably, but in my mind, it was best for her to unwind and refocus with her mother. Is it possible that something similar happened in your situation? You did mention that your daughter was really upset and scared.
It's also possible that they kicked her out maliciously, in which case, I wouldn't be going back to that gym.