I'm trying to figure out where these villas are. "The Plantation Club" is the clubhouse building where you tee off for the Ocean Course and Sea Marsh Course and also has pool (not sure who can access this pool other than property owners), bike rental place, clubhouse restaurant. It's also across the street from the entrance to the Sea Pines Beach Club, which has a restuarant, life guards, bathrooms, a couple of shops. At times they use the Plantation Club for overflow parking for the Beach Club, and I'd call that a 5-10 minute walk, to cross the street, walk through the BC parking lot and hit the sand. In gereral it would be a good location for mostly beach. You can also count on some activites being available at the BC for you little ones to do, such as the kites. These take place most afternoons and have a small fee involved.
However, how old your little ones? If they are very young, you may find it easier to hop in the car and just park at the BC rather than carry all their beach stuff in the hot sun for a 10 minute (which may be optimistic) walk. Will you stay all day at the beach? Do you guys sunburn easily? Personally, I like to do the beach in smaller doses, and especially like the later afternoon hours the best. It's not as crowded and the sun isn't as strong. In which case, the additional activities (playground, tennis courts, children's show each night, pier, lighthouse, boats) and ease of eating places would make Harbourtown a better choice. HT parking is usually worse than BC parking (when comparing their busiest times), and since the distance is only 1 mile, you may not mind getting to the beach by driving or even taking the trolley (which is free and my kids like to ride around on it for fun).
Sadly, nothing is as all inclusive as Disney's HH activites! So take your pick---all beach, choose Plantation Club, for beach plus other things, go with Harbourtown.
Finally, don't forget to take your kids to Lawton Stables, bring some carrots, maybe do a pony ride. Visit Sea Pines Center and eat at Truffles or get an ice cream. Rent bikes and bike to South Beach. These are a few of my favorite things....