Wow, I've been to Sea World SA many times and did not even know about the cabanas!
Sounds like a good deal to me, considering the ages of your kids. You don't want to have to be scrambling for a "spot".
Honestly, I'm not sure your kids will get a lot out of the Sea World experience at such young ages.
They'd probably be just as happy splashing around in a hotel pool! The best place I've found is The Hyatt Hill Country down the road from Sea World. They have a lazy river, a huge pool and a kids "camp" facility. It's a great relaxing place for a vacation. Sea World is fun, but it's a lot of work with small children, so not so much fun for mom and dad!
While you are there you may want to take them to the SA zoo. It's a good one.
There is a great little train through the park . Also the Witte Museum has the HEB tree house, much like our Science Place in Fair Park. I took my little one there a lot when he was two and three and he loved it.