Well the day of the party arrived and I just finished the shark costume in time...and it fits -yaaaay!
DS is pretty happy with it and even though its not 100% perfect it fit the bill for the job at the time :)
If I ever made this up again there are a few things I would do differently, for one I'd probably use some sort of iron on pellon throughout to give a bit more body to the costume and maybe revise the bottom part of the hood/shark mouth.
Also crappy fabric is crappy fabric...this was "pop top" poplin (I think) and the cheapness really shows!
Luckily DD is a couple of sizes smaller than her brother so we may get a little more wear out of this one :D
Happy sewing / costume making! ;)