For context- I live in Charlotte and have since the year 2000, so I feel fairly qualified to speak to the NC/SC beach scene.
If you want low key and family friendly- do NOT go to Myrtle Beach. It is the New Jersey of South Carolina beaches- sick bars and boardwalks, concerts, shows, etc but extremely trashy and the complete and total opposite of “low key”
I’d recommend two beaches I visit frequently- the first being Hilton Head Island, SC (Incredible for families, known for family beach vacations/trips) and the second being Wilmington/Wrightsville Beach, NC.
DM if you want me to give some actual planning info and whatnot but I cannot recommend Hilton Head Island enough. Getting married there this fall, fiancé and I both grew up going there as kids with our respective families. You can fly into HHI and it’s ~10 mins to any of the hotels/houses on the island. Stay in Sea Pines and go to Harbour Town, climb the lighthouse, visit the playground and shops, go to the many beaches and eat at Salty Dog Cafe. It’s a phenomenal time.
For Wilmington, you fly into ILM. The beach is Wrightsville Beach and you can get houses either on the beach area or in Wilmington where there is more food/entertainment. Also really enjoy visiting there- very popular spot for people from CLT to go on a long weekend.
Hope that helps.