I am not sure how little your DC are, but we were there last August during tropical storm Faye and it rained all but one of our days. I would like to think if anything, we are experts on panchos. I don't know where to buy them for cheap outside of the parks, but I don't recommend panchos for little kids. Our DS (4) had a hard time with it (kid-sized pancho from Disney). In the end, after many many days of rain, I really just wish he had a nice snug raincoat with hood, really just a raincoat. This would have been much more comfortable and dry for him.
At any rate, good luck and I hope it doesn't rain!