Lesson #1: Pony bottles are adorable. (Yes, I knew that already, but it's worth repeating and putting in bold.)
Lesson #2: If you are going to replace the white sugar in the recipe with brown sugar, please measure brown sugar correctly. The correct way to measure brown sugar is to pack it. What happens when you don't pack it? You end up with less sugar, and a root beer that is not nearly sweet enough.
Lesson #3. Don't use champagne yeast. Yes, the old-timey root beer recipes often call for champagne yeast to create the carbonation. The problem with champagne yeast is that it can live at very high pressures and your root beer can continue carbonating far more, and far more quickly than you need it too. Even after refrigerating, our root beer continued to carbonate, to the point where we wanted to finish up the batch sooner rather than later, or risk another terrifying episode of exploding glass bottles. Next time, ale yeast.