We have stayed at both Sabal and Royal Palms in the past few years. The world center pool people like you to get your arm bands from the Sabal desk, but there is no charge. When we were at Sabal, it was a full and a twin in the second bedroom. It's a unique set up with floor to ceiling windows overlooking the golf course- our kids loved it.
There ARE kids' activities- sometimes you have to go over to Royal Palms, but we had a ball. World center activities are also open to you, and they have a lot going on. We held a gator from Gatorland, did salsa dancing, etc. The lasar show at 9pm is neat at the World Center pool. If you are there over Labor Day, watch for fireworks at World Center!
Our favorite Marrriott in Orlando is Cypress Harbour, but Sabal Palms was a great experience. Be sure to use their free shuttle if you don't want to walk to Royal Palms. And check out all the pools. With only 80 units, we often felt like the Sabal pool was private for us