Another vote for McKinney COE. It's about 3-4 miles off I-75, maybe 20 miles north of Marietta. This is really a nice campground. No pool but there are two nice beaches. Many sites are on the water. This campground fills up fast in the summer and on holidays. There are usually NO walk-ups from mid-day Friday-Sunday spring, summer, and fall.
We have camped at Red Top Mountain SP and it's a really nice campground, too. The roads are twisty and some big rigs have a hard time navigating them. There are no RV sites on the water. There is a large swimming area within the park but no pool. They also have good ranger programs and bluegrass concerts on weekends during the summer. If you can be there during their Iron Pour weekends, your kids would love that.
There is one RV park within Marietta city limits, Atlanta Marietta campground. It's off the Hwy 120 loop in an area known for high crime. It's rather run down and has a LOT of permanent residents. I would not camp there with children, EVER.