No, it is not polite for a young child (or anyone, for that matter) to say "you're so fat" or make any comment about someone's physical appearance, especially if it is meant in a negative or hurtful way. Making such comments can be hurtful, offensive, and disrespectful.
Teaching children about kindness, empathy, and the importance of treating others with respect is essential. It's crucial to help children understand that commenting on someone's appearance can be hurtful and that we should focus on treating everyone with kindness and acceptance, regardless of their appearance.
If a child does make an inappropriate comment like that, it's essential for parents or caregivers to address the behavior immediately and use it as a teaching opportunity. Explain to the child why such comments are hurtful and encourage them to apologize and learn from their mistake. Reinforce positive behavior and help them understand the importance of empathy and treating others with kindness and respect.