Thanks coco for the Chinese translation. I did a translation to English very quickly. Hope it helps those who don't understand Chinese!

Here goes:
Dear Raimu,
I've known you for four years already. When you were one, Mama gave you the book 'Egao de Yukou'.
When you were just born, Mama said to you many times 'No matter what, we must face it with a smile'.
When you were just past one, you slowly grew up. When you were two, you were still unable to communicate with Mama and the people around you.
There was a period of time when you cried everyday. The only thing Mama could do at theat time was to smile and hold you tightly in my arms.
When you were three and starting learning sign language, Mama gradually started to understand what you were saying and Mama was very happy.
Using your small body, trying hard to express yourself, seeing you like this really makes Mama feel very touched.
Four years old.. looking back, although there has been many changes, but your smile has always been the same since you were 3 months old.
It's the best smile! Mama loves your smile.
For the sake of not letting this smile fade away, Mama will work hard.
Mama told the story of your hearing disability to the world - in this world, there are many other children who are also hearing impaired like you.
Or they might be suffering from the pain of hunger, or the pain of having no shelter, there are mnay kinds of children in this world.
When you came to Mama's side, when I see your smile, it makes me want to see more children smile in this way.
I thought it would be great if I could do something to bring smiles to other childrens' faces as well.
Because of this dream, Mama decided to tell the story of your hearing disability to the public.
Mama told everyone that even though one might have a hearing disability, it's only a trait, that is what Mama realized after living with you for four years.
Everyone is the same, there is not much difference, including children - Mama wants to let everyone know this.
Now that Mama has made this decision, Mama will take up the responsiblity and work hard to send this message, in the future as well...
When it comes to the day when Mama can finally share this story with you, Mama will be very happy.
Although you're only four now, you already have many people who care for you and love you.
Because your smile has given strength to many people, Mama will help you record all of these down first.
With these feelings, we will walk this path together.
Raimu, thank you for coming into this world.
Thank you for choosing to stay by my side.
Hopefully one day, you will use your voice and call me 'Mama'.