I would move the packed boxes to a storage facility, to a garage up high.. somewhere she can not reach.
I also would have a LOT of conversations about moving, feelings and fears she may be having. Go to the library or go to the book store and purchase picture books about moving,
Find out what it is that is really bothering her.
When ever our daughter would react the way you are describing, it meant we needed to stop and figure out what the real fear, assumption or thoughts were going through her head.
Sometimes it was totally not what we were assuming was going on and easy to fix.
Is she worried she will never see these friends. She will miss her room. She likes the routine of the way you all were living. Then help her figure out how to feel secure about the move.
I find that children this age, LOVE to be "Big Helpers".. "Dad is packing up his books. Could you bring over all of the books that are this big.."
"Ok, great job, now can you bring me all of the books that are this size?"
Or who do you want to invite to the park this afternoon so you can say good by. I will take pictures and get their address so you can send them letters!
Lets go to the store and pick out note cards so you can write letters when we get to the new house!
Lets take a picture of every room in this house so you can remember. Lets take pictures of the playground Your favorite ice cram shop.. (you get the idea. She needs some control. She needs some understanding and she needs you to have some empathy, by saying. I understand you are feeling sad because....
"I understand you will miss your room. Maybe at the new house we can paint your room the same color.." Or "For the new house you can pick out a new color. " Only offer what you are willing to follow through on.
With your wife's death, it could be deeper concerns for your child. So figure out what that is. .. Many times it is pretty simple to me and you, but to your child she cannot figure out a solution.
Moving is hard for most children. This situation is amplified because someone is missing, that should be there.
Hang in there.