This letter is an apology to Santa for having behaved badly. It should be sent as a pre-emptive strike before Christmas.
I know I haven't been a very good <boy/girl> this year, but Mommy says that sometimes you'll change your mind about kids if they ask real nice, so I'm writing you this letter to say I'm sorry for being bad. I'm working very hard to stop <bad things you've done>, and trying to study harder and play nicer with my friends. I won't ask for any toys because Daddy says that would be greedy, so if you would just bring me something nice I would be very glad. Thank you, and I hope you can see that I'm being as good as possible.