Moved from Northern VA to HHI about 6 years ago, bought a house here in Sea Pines 3 years ago and wouldn't even consider moving back to NOVA. Love the leisurely lifestyle, beach, fishing, kayaking and all the other outdoor activities HHI has to offer.
Sea Pines in March is fine as the summer season is when we get flooded with summer vacationers. There are a couple Marriott properties in Sea Pines but no "hotels", you could also look into Sea Pines Resort as they have different lodging options. I would just rent a house or a villa that's your best bet, try VRBO, Resort Quest or there are several different agencies that rent and manage properties in Sea Pines. I think you would enjoy a home or villa as opposed to a hotel and the prices might surprise you compared to the oceanfront hotels.
Things to do in Sea Pines would be Harbor Town would be the light house, the pier and they have a cool little park for the kids (nothing special but the kids love it). The run all types of Dolphin/Nature tours out of the marina and I believe have a Pirate tour that your son may enjoy. South Beach is a cool, everybody hit's the Salty Dog. I would check out the Coastal Discovery Museum which is on the North end of the island and off island they have a place called The Sandbox which is an interactive children's Museum.
Would definitely make a day trip to SAV. Steer clear of river street (tourist trap) and just walk around the historic district and all the squares (Monterey square is one of our favorites). I would recommend walking on Jones street then make your way towards Forsyth Park. Ms. Wilkes Boarding House is the spot for lunch if you don't mind waiting in line or the Green Truck Pub has great burgers (suck it In & Out). The Bonaventure cemetery is about a 10+ minute drive on the east side of SAV and is one of the most unique places I have seen. The kids may get bored but the place is amazing.
Dining option on HHI -
Palmetto Bay Sunrise Café - great breakfast spot
Charbar - great burgers
Sea Shack or Kenny B's - the top 2 seafood spots, both are holes in the wall but good stuff
Fiesta Fresh - best Mexican on the island
Lowcountry Backyard or Annie O's - southern
Fat Babies - best pizza, Mellow Mushroom is a decent choice as well
Red Fish, Vine, 843 & CQ's - $$$ but great meals
Fubar - Not sure what your field of employment is but I would make sure you either have a job lined up, be prepared for a decent pay cut, or have $ in the bank. If you can work remotely great but not many opportunities on the island unless you are in the Hospitality or Golfing industry. And no disrespect but Nick's is probably one of the WORST eating options on the island by far, locals steer clear of that place.