Sounds like a lovely evening at Epcot.
I remember that some of the storytellers are known for being better than others, so you might want to look up the top recommendations if you won't have a lot of time for them. I think that Germany was a pretty good one, and you'll be right there, so that might be an option.
I think Biergarten is a good pick. The show will keep the kids amused and the food is good and plentiful. I'd like to get back there someday.
Wow, so NPH is narrating that night, huh? I bet it will be packed! Not sure it will help if there are crowds, but there are some tables in the outdoor seating for the American pavilion that you can see/hear the shows from - ones near the edge, closest to the theater. Unless you get there early, they'll probably be taken, but something to keep in mind in case they're not and you want to stop and watch/listen.