Have I gone too far?

My niece is a sweet-angel-baby most of the time, but ever since the day she stepped foot in school, she's taken a turn for the worse. Most comments coming out of her are negative these days. We know it's from influences at school combined with her clumsy attempts at humor but it's become annoying and even hurtful. Today her Dad was her target and he told her how upset he was with her. She's only 6 years old so we doubt she really understands how much words can hurt people. She's never had a loved one say that they hate her but it's apparently all the rage at school. My bro thought a note from Santa might help, so I came up with a legit looking email from Santa (it even says "From: Santa Claus" in the heading... awesome!) the note said:

Dear A,

Because of the following incidents, you are on my Temporary Naughty List:

• Saying hurtful words to your Grandma
• Writing mean note to your Brother
• Saying hurtful words to your Daddy

I know you are a nice young lady so I will make a deal with you, if you can be good from now until Christmas I will put you back on my Nice List.

Please be a good girl.

Love, Santa

I sent it to my brother's email address and instantly suffered from terrible guilt. I spoke with my bro and we agree that this is just a phase and that she's really a sweet kid. On the other hand he thinks she may have to finally learn a lesson. I have no idea if he'll be able to show her the email or not. I don't think I could. I can write 'em but I don't think I can deliver 'em.

Have I gone too far? Oh, the guilt!