I think this might not be a feasible proposition unfortunately. Even on the Great Allegheny Passage, which is very well developed, there are stretches for 10-20 miles or more where non-flushing toilets are all that are available. There's not a lot in either direction from Ohiopyle or between Confluence and Rockwood and the entire stretch from Frostburg to the Continental Divide is on a mountain, essentially disconnected from any amenities. Would it be possible to do a shorter day trip locally as a trial run, to see what kind of distance is tolerable between access to regular toilets? Alternatively perhaps an option exists for flat road riding near the coast but also regularly through coastal towns? I have biked through Delaware and along stretches of the Outer Banks and both were very flat for the most part, but your family would lose the shade and lack of motor vehicle traffic that a dedicated bike path offers.