We go every year to Captiva usually early summer or late fall. We go to Disney probably every year to every other year. . . they are night and day if you want to find any resemblance in resort life. Southseas is, in my opinion, the closest to a resort experience, but boring compared to almost any Disney style resort, for kids. We don't stay there because I think it's overrated and unnecessarily expensive, especially since you can find a bunch of great places for 20% to 50% cheaper on the island that are just as nice. If you go to Sanibel, savings are even better, but Sanibel is too crowded for my taste and the sunsets are no where near as fulfilling. . .I'm admittedly spoiled.
But the resort, or the island for that matter, is not boring if your kid can be cool with a beach, amazing shelling, ridiculously good fishing, exploring nature (Ding Darling), sailing, kite surfing at the East end of Sanibel (maybe too young?), kayaking, parachutes, sea turtles, alligators in the reserve, birds, lots of kids on the beach. Dolphins abound usually every morning. As a parent, I can't imagine a more exciting environment for a kid to explore and a parent to relax. Kids approve. My daughter likes Captiva more than Disney. Although it's a close one...
The restaurants on Sanibel are overall better than the relatively few on Captiva, but Captiva is way more relaxing, and if you know which places to go, Captiva has it all. Be sure to stop at Target or some grocery store and stock up before you cross the bridge. Once on Sanibel, you'll pay about 10% - 40% more for groceries and about half the variety.
The ocean is best to swim when calm. In the summer, you will experience more churn, which means chances for rougher water. Although you could also have a whole week of calm, perfect water.
I do not let my kids swim during rough water not because of it being overly rough. Riptide or >3' waves are the only time I'd normally care. Rather because of sharks, which are abundant. . . Now that's said not to scare you, because sharks are everywhere along ALL of the Florida coast, and t they have absolutely no interest in biting or attacking humans and in clear water, will go a mile out of the way to avoid you, but in rough water, they don't see you and can bump. . . I fish every day off the beach and also the sound there and see sharks all of the time. They are super spooky of humans, so clear water no worries, but I've watched them in rough water get within a few feet of unaware swimmers. . . anyway, if there was one thing that's a turnoff, it would be rough water. That said if you go to Southseas, no big deal, many pools, all nice.
Anyway, I love Captiva and happy to opine further if needed.