When we had someone step right in front of our kids while waiting for a parade last summer (not at WDW), I said very loudly, "Ma'am, I'm sure you can't possibly be this rude on purpose, but you've stepped right in front of my children who've been waiting here for half an hour already." I basically called her rude right to her face but very politely. She was so embarassed that she not only moved out of the way, but left the area (because everyone else heard she was being rude?). If she hadn't moved out of the way, I would have replied, also very loudly, "Oh, so you are being this rude on purpose?!"
That's the line I'm going to try on our next trip while waiting for parades, in lines, etc. Hopefully it will work then, too.