Free and easy with kids
You don't need to book much. Waterfront Park has the Pineapple Fountain, which doubles as a splash pad on hot days, so bring a towel and let them run. The Battery and White Point Garden give kids cannons to climb on and open lawn under the live oaks. The City Market is stroller-friendly and good for a snack and watching the sweetgrass basket weavers. And the Angel Oak on Johns Island, a free, 400-plus-year-old live oak with limbs that rest on the ground, genuinely impresses kids in a way photos don't.
For a free beach morning, Sullivan's Island is the calmest and least crowded, about 25 minutes out, with a lighthouse and gentle surf. Folly Beach is livelier but harder to park on a summer weekend.
By age
Toddlers and preschoolers: lean on the waterfront, the carriage ride, and Magnolia's animals. Skip long house tours, which test small patience fast. Bring a carrier for the cobblestones.
Ages 5 to 10: the harbor and dolphin cruise is the sweet spot, along with a beach day and the Pineapple Fountain. Fort Sumter (booked separately through the National Park ferry) works if they like history and boats.
Tweens and teens: they can handle the deeper history, including a daytime carriage or walking tour, and most are fine on an early ghost walking tour by 11 or 12. Kayaking Shem Creek is a good outlet for older kids with energy.
To turn these into full days, the 3-day itinerary has the kid swap built in: the aquarium and the USS Yorktown in place of the harbor cruise, and Magnolia over the other plantations.