never gotten one--don't see a need. We get 4 loungers and an umbrella 2nd row up from the beach near where the snorkeling starts. If kids were younger, I would get chairs/umbrella on the 1st row to be within arms reach. We have never had an issue gettting chairs/shade at peak, crowded times, like Easter. We take only nonvaluables with us and just leave in a beach bag. We rent a couple of floats or rings and bring some sand toys from home. Our kids play in the water/sand or we relax under our umbrella and then get our free lunch over at the BBQ. We also get bikes and ride around the island for an hour. We've been 4X and always had a great day and never gotten a cabana. For a 7 and 3 yr old, consider taking goggles or an eyes only mask and have them float with you while you snorkel and put their face in to see fish. Less intimidating than trying to snorkel. At those ages, our kids really liked the kids clubs on the private island a lot. DH ate a quick breakfast and got our chairs for the day and came back to the boat (the 1st cruise, after that we just met him at the same umbrella--still chairs/shade at 11 am), checked the kids into the kids club, then we snorkeled, relaxed, picked up the kids, ate lunch, then did family time at the beach for the afternoon. Total paradise, can't imagine needing a cabana. We have rented cabanas on other islands, like Grand Cayman at crowded times, so we would definitely rent one if I saw a need for privacy, space, shade, etc.
For the Bahamas, I would sign up for an excursion--whatever you like. Many like Blue lagoon. We usually go over to the Sheraton (now Melia) and get a day pass on our own. Many also just walk to the British Colonial Hilton and get a day pass. I would spend my $$ on an excursion in Nassau rather than a cabana.
make sure to check the navigator about when to get tickets to character meets. I almost missed it--the 1st one was 2-4 on embarkation day.