I always thought that everyone at the table would be charged, whether they eat or not. But on our last trip in Feb. we went with a large family group. When we ate at Biergarten I fed my extremely picky DS 11 before we went because I knew he wouldn't eat anything there, but I was fully prepared to pay full price for him. My DSIL checked in our party, and without my knowledge she told the CM that my DS wouldn't be eating, and they had no problem and didn't charge us anything for him, not even for his lemonade! I never would have even thought to ask.
So I guess you could go ahead and ask about it at check-in, but I'd be prepared to pay full price.