I'm thinking of taking my niece's keychain
Why would you steal her keychain?
I don't care too much about her feelings
You shouldn't take things away from people, and especially not from children. It may just be a trinket to you, but it sounds like your niece really loves that key chain. It's cruel to willfully upset a child like that.
Hey, I don't play fair.
Don't you care about your niece's feelings? You should just let her have it.