While I am sure that some excellent advice has already been given, I would like to offer something for your consideration. This route, from Erie to Pittsburgh, Morgantown, and now the Montour, The GAP and points beyond has been around for many years and has challenged cyclists, past and present.
While contemplating this ride myself, some years ago, a wise man once said, "Hell, if you're going to do that ride, why not do it in Ohio? There are a lot fewer hills!"
While our NW border with Ohio seems like an arbitrary straight line, drawn by some surveyor in colonial times, when you are cycling, there are many places where shortly after crossing into PA, the hills begin with a vengeance. The winding roads with poor sight lines and more traffic also start to occur in the Keystone State.
If hills and winding country roads are your thing, stay in PA. I'm thinking that especially with a two-year old, it might not be. Ohio has many roads that are straight as arrows with fewer hills and less traffic. From Erie, go about 20 miles West, over to Conneaut, Ohio and start heading South on the several North/South backroads you can find on the map.
Near East Liverpool, Ohio, you will have to start heading East and will then encounter some big climbs, but it is possible to stay on backroads in West Virginia and PA. You may need to get on PA-18 for a bit to Burgettstown where you can catch the Panhandle Trail and head east. At McDonald, PA, the Panhandle connects to the Montour Trail (Trail of the Year this year, hence your desire to see it, I presume).
Not sure what Vannevar told you privately but his reservations about the Montour Trail might be due to the fact that in it's Southeastern portion, the trail is not finished and you will need to go on some roads to connect the various unfinished sections with finished sections. Near Large, PA, you would have to take on Route 51 (the bane of the existences of all cyclists who ride South of the city), for a few tenths of a mile. There is a good strategy for heading South on 51, to minimize risk but coming North, you either walk on the grass or get out into a longer game of chicken with fast moving traffic.
As Buffalo offered, if you are comfortable letting anyone know of your itinerary, I'm sure there are any number of cyclists, familiar with the area, looking for a good excuse to go out for a ride.
Here is a possible route: https://ridewithgps.com/routes/3683045
In any event, it sounds like a great trip. All the best.