I rode the Cape Cod Rail Trail last year, starting from the trailhead parking lot in South Dennis on Rt. 134, all the way to the other end at Wellfleet. The entire trail was repaved 3 years ago and it was still in very good shape when I went, and obviously it was pretty flat (virtually no hills). My cyclometer says my total round trip was 42 miles (it might be off).

There are various bike shops along the rail trail with rentals available, but I'm not so sure having kids ride such a distance on rental bikes is a good idea.

The other end of the Rail Trail at Wellfleet is just a parking lot in the woods with a connecting street to Route 6.. Not much to see there unfortunately, unless your Boy Scout troop are willing to brave Route 6 (2 lane highway with varying shoulder widths) to head further up Cape Cod into the town of Wellfleet Center itself.