I think the main thing is to show kind behaviour and they will hopefully follow, but it may not be seen that way by other. As an example, I have 4 DC.
DS1 - has always been well mannered, helping others and people commented on it to me. Not so kind to his siblings, especially DD1.
DD1 - is kind to her friends, but unhelpful at home and can come over as quite rude when out and about, this is getting better at 16 but between 8-13 she could be quite short with people. However, she does have some autistic traits and we are hoping for an assessment now she is at college. So some of this might be because of her anxieties in new places with new people. At school she was a mix.
DD2 - she is 11 now and has always had a kind nature, helpful at home and school, loves her friends and likes to help others. She has raised money for charities and considers her impact on others, looks after and plays with her younger brother since the minute he was born. When she was 2.5 we had another child who was stillborn, and despite her young age offered me great comfort as if she knew I what I needed, this has continued and she was given the kindness cup from school in Uear 5.
DS2 - he's 7 and has his ups and downs, still kind and thoughtful but can be a little unpleasant at times, but this has been a recent development and I'm hoping it doesn't last.