I would file this is the "Annoying, but not rude" category. I don't think it's reasonable to expect to go through life never being annoyed by other people. Sure, we should try to mitigate our impact on those around us, but it's not always possible.
It sounds like this little girl was exhausted and I don't see what the mother could have done to make it better. It's very probable that not getting on the bus was not an option, and nagging an exhausted kid is useless, so what was the mother supposed to do?