D.B.
We used to go to Duck near the Outer Banks. We had little kids in the family (no pool). The riptides and undertow were really strong - honestly, it was too hard on everyone. After two years of it, we decided that we needed to go farther down south to Wilmington to Wrightsville Beach. That water is so much easier.
Hilton Head is nice, but VERY crowded. Have you looked into Kiawah? That's a less crowded place.
To be honest, lifeguards are great, but you have to be the one lifeguarding your own children. They have throngs of people to watch. Having a calmer beach with the green flag out more often than not is what is most important.
Have a lovely time, wherever you choose!
Dawn