Can you tell me more about the Pirate Museum? What's the name, how much for admission?
I don't know about this particular museum but I think one of OC's best-kept secrets is the shipwreck and pirate museum on the second floor of Sea Shell City (just be careful not to say that too fast!

Twinglemum, my youngest was about 5 when we went to Frontiertown and he LOVED it! It was a lot of fun. It's close to Assateague so you could easily combine the two. There is also a waterpark just outside Frontiertown and I think you can buy a combination ticket that lets you do both for the day.