I have seen a few stores that sell hoodies with the Tardis designs on them like so:

I was thinking about doing something similar for my friend, but I almost feel that it will be easier for her to just buy one of the hoodies and a matching pair of sweatpants, but I can't find any that would fit a three-year-old.
My other choice would be to use a box and either paint it or make a Tardis cover for it. I would end up with something like this:

And with either costume I think this hat is appropriate:

My question for you, what do you think would work better for a young child? I think he would be more comfortable in a hoodie and pants, but the box outfit would be adorable. Also, do you know where we could find a kids sized Tardis hoodie?