You should look into Charleston. There are not many "beach front" hotels, but you could try The Tides
http://www.tidesfollybeach.com/. Foley Beach is a very neat beach community that has become very popular. The Folly Beach pier is right there, and the Morris Island lighthouse is down the road (after a short walk in the sand). The area right in front of the Tides hotel can get very crowded though, and a bit rowdy as it's popular on Sat-Sun with a drinking crowd. Charleston has a ton to offer for kids and families and is very tourist friendly.
Myrtle Beach has many "kid friendly" beach front hotels, but they will be crowded and there is not much to do in Myrtle except for very tacky, touristy things.