The letter includes responses from Angela and the resolve by Cadence that though she was born with autism, she wasn't born "bad".
Angela admits she was reduced to tears by her daughter's words.
"I have happy tears that you know what is true; and I have sad tears because there are lots of people who don't know what is true," Angela wrote.
The post has so far generated more than 100 comments, applauding both Cadence and her mother.
Angelica Mauritzon wrote: "Dear Cadence. Their is nothing wrong with us. We are just a little more special and put in to this world to teach others to be more understanding and not so closed minded."
Another said: "Thank you for sharing. My son has autism. It is a very important message that you are sending."
Others asked if they could share the post with their children's teachers.
Read more: 9-year-old's 'girl code' answer to math problem
- nzherald.co.nz