The KOA has the most stuff to do for the kids. They have new owners and they've done a lot of good stuff out there. Just remember that the KOA is 30 miles from the fairgrounds.
Anything up in the Johnston / Polk City area around Saylorville Lake is good. Not any organized kid's stuff there, but good "nature" parks.
The RV campground at Adventureland in Altoona might be worth a look to you. You are 7 miles / 20 minutes to the fairgrounds from there.
There are also a bunch of County parks in the area with hookups. Check out the
My County Parks website.
Timberline Campground in Waukee is a decent place too. You are within 10 minutes to a huge mall and lots of restaurants. It is ~14 miles / 25 minutes to the fairgrounds from there.
2011 Born Free 22' Class C Rear Side Kitchen