I have a confession to make. I'm not sure I know any Jimmy Buffett songs other than Margaritaville.

In the 70s and 80s I listened to my parents' music which was mostly country and ABBA. So I am definitely not picking Margaritaville for me. It's not really "for" the kids either, but more of a combination of factors that work for our family. Of the restaurants at Universal, this is one that gets good reviews for the food, but is not too pricey. Emeril's and Paula's restaurants are expensive. Hard Rock Cafe is boring (to my kids and to us who've BTDT at many other locations). I didn't look too long at the CS available inside Universal. There may be plenty of options. I liked that Margaritaville has "local" foods, such as conch fritters and jambalaya. But it's obviously casual and loud so we won't be stressing about our kids sitting still and being quiet. I think there is music every night, sometimes a band, and I've read reviews that mentioned balloon artists and such entertainers for the kids.
Morning, Melissa 
I nixed it more after looking at the menu prices and realizing it would be a "me" thing and not a Piper thing. I realize the prices aren't terrible considering some of the prices at other places, but it's still more than what I'd like to pay when I know I can do less expensive that day and Piper wouldn't care one way or another. We don't have anywhere like Rainforest, TRex, Margaritaville anywhere near us, and of those, I think she would get the most out of Rainforest, so we'll do that on our AK/DHS day.
I tried to see the "beach" in pics, on google maps etc, but I can't find any. I wouldn't want anyone disappointed if they were expecting an actual beach.
Deciding on the dining was the hardest for me (and of course, could still change) but you just have to decide what factors are most important to you. Things that I considered when deciding were: Characters, unique themes not found at chains at home, unique menus that aren't too exotic for the kids to try, price, location, availability. It's a tough decision, but of course, some days/nights you can just leave open, maybe have some ideas written down as backup and then wing it! Good luck!