With 5YO twins and a toddler, I'd probably lean more towards Myrtle Beach Travel Park than Pirateland.
In Pirateland, the pool and the beach are quite a hike from each other, which will make one or the other an issue to get to. In MBTP, there are plenty of sites close to both. Not sure about the configuration of any of the other parks.
For your first experience there, trying to keep track of the twins outside in a dense park can be quite an ordeal with some of the idiot go-carts zooming about, which is another reason why I'd lean to MBTP over others - one less thing to worry about.
As someone else said, many, many mini-putts around. Multi-levels on top of it. Will be quite an experience for the twins.