We like Old Mill Stream in Lancaster. It is very well maintained and friendly. Located right in the middle of all things Amish. It's on RT. 30, which is the main east-west route through Lancaster County.
There are a lot of campgrounds in eastern Lancaster County. Beacon Hill, Country Acres, Lake in Wood, Spring Gulch, Yogi Bear Quarryville, and White Oak are all reviewed in the Mid-Eastern section of the Campground Review section of the forum.
Mack