DD5, who is also a music lover and dancer, reports that Biergarten was her favorite experience of the trip, and we ate at both CRT and Akershus (and she's also obsessed with princesses). The band is so much fun to dance to, sing with, etc., and they really encourage the kids (and adults) to come on down to the dance floor. She met a bunch of other kids and they all danced together, which was just delightful to watch! Great memories made that night.
It's our family's favorite "fun" restaurant and our one absolute must-do! If your son loves music, I think he'll love the experience.
To answer about the food, DD5 is not at all picky, but she doesn't eat a large quantity of food as a rule. She did enjoy the food quite a bit, but a little was all she needed to be full. DS2, who eats like a college football player, put away several plates (not an exaggeration). DW and I love the food, although to be fair we greatly enjoy German food as a whole.