Hi busterbrown,
I'm also a metro-Detroiter who travels with my wife and 6 year old son. One of our regular local stops is at the Frankenmuth Jellystone campground. It is very clean with many activities for young kids. There's an indoor heated pool and they will have an outdoor pool by mid-summer. We typically go for one weekend in the summer and one weekend in October as they have great Halloween weekend activities throughout the month.
You will love your Ludington SP trip. We've gone there every year since we bought our trailer. Bring bikes as they have plenty of areas to bike/hike/fish around the park. If you have a bike trailer, you can load the kids up and bike to the lighthouse also.
Enjoy your new trailer!
