How Full is Your Bucket (For Kids) which is based on some proper grown-up self-help book I have not read. It teaches kids to think about how they are making others feel and be aware of how they are feeling and most importantly how those two things interact. How people can lash out when they are emotionally drained and how helping others makes you feel better too.

and
Each Kindness This is pretty heavy and it will leave everyone feeling pretty sad at the end. I still can't read it without getting teary. However that's the point. It's meant to make children reflect on how they treat people. The story is told from the point of view of a girl at school. A new girl joins the class and it's clear she comes from a family which is struggling financially. The new girl tries to make friends but the main character and other girls snub her until she's forced to reflect on how she's been treating the new girl but before she can do anything to make things right, the new girl moves to a different school and the main character is left to deal with that regret.

These were both great ways to start discussions with my 5-year-old daughter. Another one which was good for talking about feelings was a picture book adaptation of Inside Out (the Pixar movie). She's found "who is driving the console?" a good way to frame what she and other people are feeling.

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Reddit mentions of Each Kindness (Jane Addams Award Book (Awards))