For Brookfield Zoo, the food at this outdoor eatery is very good. But I emphasize: that is compared to food at Brookfield overall. In general, the food is pretty gawd-awful all over the park--you are MUCH better off bringing your own grub. However, when the Grotto opened last season, my husband and I were pleasantly surprised by the quality of the food, the service and the fact that it serves wine, which you can't find anywhere else in the park.
Food: Comparable to your average chain-restaurant (think TGIFridays or Chili's) in quality. It isn't horrible, but Michelin isn't offering up any stars any time soon. Selections include sandwiches, salads, a few appetizers. The menu is small, but that's a good thing--they can focus making the few things on it well.
Location/Atmosphere: The Grotto is located across the walkway from the old polar bear dens and is downwind of the children's petting zoo. What does that mean? You're pretty much guaranteed a constant "goaty-ness" hanging in the air. In my opinion, it's tolerable--but I imagine that's really determined on a person-by-person basis.
Service: The Grotto hasn't really caught on just yet--in fact, we feared it wouldn't open again this year (but it did!)--so there are usually plenty of open tables. This translates to attentive service by friendly wait staff.
Kid-friendliness: You betcha! Great little kids menu, great selection and since the Grotto is located outside, loud children don't seem as loud.
DISCLAIMER: For Brookfield Zoo, this is a 4-, possibly 5-star restaurant. Generally speaking, it's a solid 3.