I'm really not a fan of ponchos, my first trip to WDW was in late august / early september of 2008, and it was when Hurricane Fay decided to park over Orlando for the first 4 days of our trip. It rained, and it blew, but the parks stayed open, it stayed warm, and we walked onto every ride because most people stayed back at their resorts for some silly reason. We ditched any thought of ponchos after day 1. We're used to cold, wet weather, so aside from a few chills when we entered restaurants or other inside attractions, we just trucked through and had a blast.
The youngest person in our group was also 13.... Though I don't mind getting wet myself, I'm sure if you have little ones, you'll want to keep them dry whenever possible.
I guess the main message is... Rain won't stop the Disney magic.
Oh, and regardless of the forcast, you can expect a 20 minute downpour once a day in the early afternoon. Maybe it's just the time of year we usually go, but it sure is consistent!