Myrtle Beach might be good if you simply like the beach and pools. Granted, the water will be of the East Coast variety, and definitely different from Caribbean, Hawaii, Bahamas, and Cancun, but kids still like it. Maybe even better.
Altho we live in the east, I've been to all the places you've described, and to the gorgeous Newport Coast Villas too.......and it will be sooooooo different for you here on the eastern shores, but you'll never know whether you like it until you try it.
One thing for sure, I do indeed doubt you'll like driving to a public parking lot for all beach activity, tho. When at Sheraton Beach Plantation, that's what you'll have to do. And the parking lots, if you can even find one, are first-come, first-served. Except for North Myrtle Beach, the rest is not an upscale, "classy" place, imho, but it's fine for what it is.....beach and pools.
There are very nice places there, tho, like Marriott's Oceanwatch, which is definitely first-class. That you would like. We own both there at Oceanwatch and at SBP (SBP just for extra, less expensive MB time). We mostly stay in Hilton Head, tho, where the majority of our Marriott weeks are located. You might consider there.
The beach at MB will be broad and flat......perhaps wider than any you've seen, and easy to walk, bike, and play games.....kids love that and will never forget it. Also, no shells or rocks, so it's truly great fun to boogie-board or ride the floats, as we called it. We had the rectangular inflatable canvas-covered floats a few years ago when our kids were that age, and those floats are still my favorite.
Charleston is fantastic for adults, but young kids will be bored to tears....there's absolutely nothing there for kids......a great city for wining and dining, sightseeing, and historic things, but that's it. And Charleston in the summer is close to unbearable. Don't dismiss that fact---it's awful. You have no idea what humidity is until you visit Charleston in summer-----it has little breeze, if any, and you'll not be happy. Possibly same for MB, but at least there's water to jump into.
You want a real suggestion? Go to Orlando. Your child will love you forever, and will NEVER stop talking about it, and frankly, you'll have the time of your life too. Beyond plenty for kids and adults to do.....and the quality of things is unsurpassed anywhere in the world, period. The fun there may perhaps supersede the heat, so that's the only reason I bring it up.
If you've been to Anaheim/Disneyland, please know that it doesn't even rate on the same scale. Walt Disney World is an eleven, Disneyland is a flat 3. It's all you have out west, granted, but imho it's not clean, it's extremely small, and it's not in the same class. Just sayin' for comparison's sake.
On the other hand, WDW has Magic Kingdom, Animal Kingdom, Epcot, Blizzard Beach, Typhoon Lagoon, Pleasure Island, MGM-Disney Studios, Universal Studios (two separate parks), Sea World, and more. They are altogether thousands of acres in size, and are all clean as can be. There is no comparison. And the age of your child at 6.5 is perfect. He will not want to leave.
Only problem is that you''ll then be flying BACK every 3-5 years or so. The resorts in Orlando are over-the-top wonderful. Huge and fantastic pools galore. Maybe the "happiest place on Earth" if you ask a kid. If it were me, and you've never been to Orlando, forget Myrtle Beach and Charleston, and change to WDW.
Now if you're going in summer, remember that Orlando is also awful......tolerable yes, but really, really hot and really, really humid. If you've experienced temps and humidity like that elsewhere, like Cancun in summer, only you know if you can deal with it, but personally I would hesitate to go back to Orlando in summer unless it were my only recourse. School being out would be the only reason to do it.
If so, then fine.......but expect long lines, strange languages, sweating like you cannot believe, and just begging for an air-conditioned building. On the bright side, you still have Blizzard Beach and Typhoon Lagoon, where there is water to jump into. It's doable, just be prepared.
MB and Orlando will have virtually the same temps....Good luck. Gotta take that 6.5 year-old to WDW!!!!!! Can't think of a better reason to fly cross-country. You should have seen our son when he was 3.5 there, and then when he was 6 (daughter then 3).....they loved it at 3.....and have been back every few years.
We're going back soon without kids for Food and Wine Festival at Epcot in Fall, and daughter is going next week for her Spring Break.....will graduate from college in May, and cannot wait to get back to WDW...also went last year....a bunch of friends in Orlando and WDW for a week, two years in a row? -----can you imagine?