C.M.
I am sorry to read your going through this. I've been there.
But I do agree with Love R; file for legal separation to protect yourself, get copies of all financial records, cancel all joint accounts, etc.
Most importantly you and your husband should seek the assistance of a therapist as this may help to keep things amicable as well as provide you with the tools to help your daughter cope. You need only to explain that mommy and daddy will no longer be living together, but that see can see and call her daddy anytime she chooses (or whatever you two decide on) and that this does not change how each of you feel about her. When she has questions make sure you listen and keep your answers very simple.
Additionally, you'll want to keep things as normal as possible for your child so if you will be moving in with your parents and if there is room to do so her room there should be akin to her current room with all of her favorite things.
Here are a few age appropriate books for you and your daughter to read together:
1. Mama & Daddy Bear's Divorce
2. We're Having a Tuesday
3. It's Not Your Fault Koko Bear
4. Was it the Chocolate Pudding?
5. My Stick Family: Helping Children Cope with Divorce
I hope you find the information and reading list helpful.
Connie