I haven't been to Myrtle Beach in 20 years, so I don't know that much about it. I do know that a group from our church goes every January to "Beat the Blahs." The wimmin shop and the men golf.
But I don't think you'd get beach weather. Even if it's warm enough to be on the beach, the water would still be vey cold. And I don't know how "active" the sites are in the off season. Virginia Beach is like a ghost town in the winter.
If you're coming that far south, and with the girl scouts, why not consider Savannah?
Good idea, especially since the Juliette Low house is there and everything. One of the girls in the troop is thinking of going to college at the Art School there. My great-niece is considering the same school.