Disneyvoice is correct--make sure you let the host/hostess know of your needs. I'm sure they'll be accomodating. Further, we ate there just last week, and they have put a few 2-top tables in open areas on the bottom tier between the viewing glass seats and the booths--it should be fairly easy to move one of those to a 4-top and get your party of 6 seated. I'd even mention that to them.
In defense of those who may not like kids coming over to view fish next to where you're seated, keep in mind that one or two kids coming over for a brief moment is one thing. It's altogether different when you multiply that times several families, all with kids wanting to do the same thing. Further, there's not a lot of room to squeeze in between some of the tables to begin with, and kids are notorious for invading others' personal space. Unlike the nice posters to this site, there are parents who give not one whit what their children do in a public setting like a restaurant, allowing them to roam around unfettered and fancy free, assuming that their kids can do no wrong. After being literally jostled by an unending parade of 3-10 yr olds (sometimes 3 at a time) attempting to get a close-up view of the tank, we finally shifted our table a few inches back toward the next table and scooted our chairs out just a bit to limit the access. We aren't uncaring or mean or grumpy, and we never said a cross word to any of the kids, certainly. They were just curious and excited, as our DD4 was. On the other hand, I think it is proper to keep in mind that although Disney is joyous place, patrons should be allowed some privacy when eating, even in a unique place like the Coral Reef.
As for the blue ice cube, I believe it's called a Blue Whale, and it is available in any kids' drink for the aforementioned $4.95. Tap it once on the side, it goes off. Tap it on the other side, it comes back on. They are cool, and our DD4 was almost as attracted to it as she was the fish in the tank.
