Going end of June with grandkids (8, 9, 11, 12). Would love to shell and look for sea glass - if any to be found. Any ideas where? Other than beach and hotel pool, need suggestions to explore with kids. Thanks.

Shelling is very good south of MB at Huntington Beach State Park. You must pay to get in and there is a nice nature walk and exhibit. The beach is good for shellling. You'll see gators and herons and other wildlife. I'm pretty sure you'll have to get out of MB proper to do much shelling. You might go down to Pawley's Island and park in the public areas (turn right when you get to the beach and go to the end. There might be good shelling there.
Not sure about sea glass.
Ripley's Believe it or Not is at MB and there is also a place your kids can swim with manta rays--it is really pretty cool.
Down in Georgetown there is a 3 masted sailing ship that they do "pirate cruises" on and it is really cute! The crew all dress like pirates--and of course, talk like pirates. They tell tall tales and let the kids pull the sail ropes. Three hour cruise around the harbor and sea there.
And just outside Georgetown is Tom Yawkey (yes, Red Sox fans) Preserve that you can make reservations to see. Good for wildlife. And then also there is the Hobcaw Barony, just over the bridge from Georgetown for another nature conservancy.
There is a petting zoo and butterfly pavilion at Brookgreen Gardens just across from Huntington Beach Park.

Thank you, Gretchen!

Make reservations for the Yawkey, Hobcaw and pirate ship ahead of time. Google. The ship also just does "cruises".