Maybe a little long for an out-and-back (consider a shuttle) but the gentle grades make this kid friendly.
This trail connects Town Park and Cedar Point County Park.
The elevation changes through some massive kettle holes provide some short hills that add a little interest to the trail and are only slightly steep and rocky. There are three road crossings but the trail continues directly across the street in all three instances (Old Northwest Rd., Bull Path and Two Holes of Water Rd.).
This trail could make a nice out-and-back, and can be shortened if the distance is intimidating. You could also set up a shuttle. Alternatively there are other trails you can link up to make a loop.