This is more of a sewing question than a construction question. I am adding an "Instant Magician" routine to my show. My wife is an incredible seamstress. Does anyone have any resources for patterns, etc to sew an instant magician set up?

Not trying to sound harsh, but you could actually just purchase the item from the originator. A quick look shows they are only about 200$
That said, especially if you are doing a dress the child as-maybe something more original to yourself,...a wizard, etc. then I would say that your wife might take a look at some of the books in the library on theater costumes for actors who must change in and out several times in one show. I would likely start at the local thrift stores for the suit jacket and shirt to start the cutting process. The sewing will actually be rather minimal, just edge finishing and sewing the layers to keep them in place int he front.
K