We stayed at Assateague Island last August for 5 nights/4 days. It was an amazing experience. Our kids were 12, 10 and 6. I can't recommend it enough.
We booked our site the October beforehand (at exactly the opening date and time. All sites for the entire month were booked within 2 minutes), so it's probably too late for you to get an electrical site for 2016. Maybe you'll get lucky and find a cancellation or maybe you can try for 2017.
I had heard a lot of good things about Frontier Town and Ocean City, apparently people really enjoy both of those areas. We drove into Ocean City one day for dinner and went to the boardwalk. Lots of touristy things to do there, but I'd never want to stay there. It was crowded and overloaded with hotels. We also drove past Frontier Town, Assateague was like an oasis compared to that place. I'm sure the kids would have liked it, but if you get the opportunity, go for Assateague.