Myrtle Beach Travel Park is by far your best choice in the NMB area if you want to be right on the beach.
None of the big three to the south have sites that are significantly (if any) larger than the sites at MBTP. Friends with kids stayed at Briarcliffe twice and wouldn't go again if you paid them. It's a membership campground, although it's also open to the public. Both times our friends stayed there they encountered members who seemed to believe that all kids should be inside by 8:00 p.m. Apache is older and somewhat rundown, and doesn't have as many kid friendly amenities and activities as MBTP.
Me, DH and Yogi (Shih Tzu)
2017 Winnebago Travato 59K