Two things you might do to make them feel easier. As a parent I wouldn't let them go alone, I'd join them. Even if it was as a driver passing them up letting them catch up and leap-frogging the entire way. But that's just my protective nature.
Alternatively, you could have them put a Garmin tracking device on you. For $300 it has no subscription fees and is smaller than a cell phone. It can track you to as pinpoint a location as a bike GPS. That's what I would want my child to have no matter what their age would be if they were doing a century alone.