I voted for Coral Reef. I've eaten at both and understand the other posters' reasons for suggesting Biergarten, but it really depends on the kids. If they're likely to be opposed to trying more unfamiliar foods or you want to reduce the potential that they'll see drunk people (like others have said, you'll be seated with other guests, so there's a good possibility of this), I'd say choose Coral Reef over Biergarten. However, if you think they'd prefer to be able to get up and dance rather than watch fish, or if they like German food or are at least willing to try it, go with Biergarten.
Here's one small thing to consider that hasn't been mentioned yet: at Coral Reef, there are places to sit down while you wait to be seated at your table, but in my experience, at Biergarten you're standing until you get seated. This is a small enough detail that it probably won't be the deciding factor, but it's something to keep in mind.
*edit* Never mind, I wrote this reply before I saw that you'd made a decision. Hope you enjoy Biergarten; my sister and I loved it when we went in March!
