Start your ride in Frankfort
I ride the Old Plank Trail on most weekends. It really has a little of everything out there - slow walkers, roller bladers, road bikes, kids with training wheels, etc. You and your "tour group" will fit right in.
I suggest you start your ride from the Frankfort Town Square. They have a parking lot, real bathrooms (which the kids might appreciate), a convenience store, and a great bike store right on the trail. On Sundays, there's also a farmer's market (kettle corn anyone?).
After your ride, head over to the Trolley Barn for some ice cream!