MOA is way too packed. We went last year and you can forget about pushing a stroller around or even walking through the mall. It was nuts and told myself that we would never go back.
I grew up in the West St. Paul/South saint paul/Inver grove area and we always got lots of candy and had fun trick or treating. Maybe try trick or treating in a neighborhood that has a school in it. I do know that the River Hills neighborhood in Burnsville/eagan border is great. There are tons of families and lots of trick or treaters. Its located on HWY 13 and River Hills Road, or you can get there from Cliff as well. There are two sides to the neighborhood, either side would be good, but I would stick to the East side.
Also the childrens museum has trick or treating too, not sure if a 6 year old would enjoy it though.
I hope you find something fun to do. Halloween should be fun!