I'm going to tell my coworker off for being childish
Why do you need to tell them that they are childish?
Because they constantly complain about minor things and get upset when they don't get their way.
You shouldn't argue with coworkers even though they are childish. Arguing with people at work will get you reprimanded.
I'm just telling her the truth about how she behaves. It's not my fault that she acts this way. Someone was bound to confront her about it eventually.
Try to just mind your own business and not try to argue with someone specially at work. We should strive to have conflict-free working environment. Arguing will just make the workplace more awkward for everyone.