A Child's Perspective On Kindness

Before school every morning we have a "hands in" prayer time. It's a tradition.
A couple weeks ago my sister was staying with the kids for us while we attended a conference. She decided that the 'hands in' prayer time could benefit even greater with a more applicable teaching time. So every day they had a different challenge to complete. Things like: find someone today that you can help, give someone a compliment today, do a good deed...etc.
My daughter is in the drama filled world of third grade. She often comes home telling us stories of how mean this one little girl can be. Or that this little girl has informed her that she is not her friend. Or even, "you can't be in our club". There is generally a story every afternoon, to the point that the boys now get in the car after school and ask, "what did (insert name) do to you today?".
As a mom it's hard to see your child upset. I have refrained from giving too much advice because I wanted to see her put into action the things that we have talked about and how to handle certain situations. We have prayed for (insert name). We have talked about what her home life could be like and that could be a reason why she acts the way she does sometimes.
Now it was my sister who was having to hear the stories of this girl's meanness. So she decides to teach my daughter about kindness. "Be nice to her", "say something kind to her", "You have to kill her with kindness" she would instruct her every day.

A few days ago after picking up the kids I could tell my daughter obviously had something on her mind. You could sense the heaviness she was feeling. She seemed at the end of her rope. She finally says to me, "Mom, Sherry says that I should kill (insert name) with kindness", and there was a long silence..."Yes, that would be nice", I replied. And with much seriousness & thought she replies back, "I'm gonna get a baseball bat and write KINDNESS on it, and hit (insert name) with it".
I found that it was best to just pull over since I couldn't see past the tears of laughter how to drive.
Where on earth do they get this stuff? I have no idea...