Try the Shannondale area of Harpers Ferry. There is a lake beach, trails, cabins, etc.

Recommend posting this on the travel forum instead!
I’m a fan of near-in WV. Nice spots 2ish hours from DC/Nova Maybe check out Airbnbs with use rights of the Coolfont resort amenities (pool, playground etc) in Berkeley Springs or perhaps Capacon State Park cabins which I think fit all your criteria though not sure about pets. Lots historic spots within short drives.
But tbh there are many such spots in the area you mention and the travel forum folks would likely rattle off a long list of ideas.

I need location ideas for a summer vacation that fits these criteria:

-good for a 6 year old, a 4 year old, and a 1 year old (nearby playground, stroller-friendly trails, family activities, swimming, maybe a beach...)
-cabin or rustic aesthetic but with electricity and bathrooms
-place for a campfire
-we want to vacation with the grandparents so we need to be able to get a larger cabin or cabins next to each other
-probably in Virginia or North Carolina or surrounding states (grandparents live in NC, we live in VA)
-dog friendly
-nearby museum(s) or historic site(s) (more for the adults)