I would be annoyed, if I only got two days to prepare for something like that.
At the moment my son (20 months) would have to go as Tigger, as that is his favourite character... :o)
How about Paddington bear, with a suitcase, a hat, a coat and a sign saying: Please look after this bear.
Then a little make up and FINITO!
I don't know what six-year olds read these days.
I was into the Disney books...
There would be Merlin with a cloak, a crooked pointy hat and a wand.
Or Mickey Mouse.
Or Arielle, but that takes a little more sewing.