Emmy is the youngest of the Walsh kids, and so she is the most protected when it comes to Nikki's tantrums and violence. She also doesn't live through much of Nikki's abuse because Nikki loses custody when Emmy is still just a little kid. That's why Matthew starts writing this letter to Emmy—so that she can know what their family went through:
As I write this, you are nine years old, too young to be told the full and true story of our family's past, let alone be exposed to my philosophizing about what it all meant. I don't know how old you'll be when you do read this. Maybe you'll be seventeen, like I am now. Or maybe much older than that—in your twenties or even thirties. (P.1)
Although Emmy lives through these events, she can no longer remember them, which is lucky for her. Instead of being weighed down by the abuse that she's suffered, Emmy can forget and move on. That's something that Matthew and Callie want for her, and are unable to get for themselves.