I've cycled north from Le Harve. I think the trip from there to Boulogne is one of the real gems of cycling in France. That coast didn't attract all the painters from Paris for nothing. The coast together with the odd wander inland for some history (Agincourt and Crecy to both world wars), can happily fill a couple of weeks of anyone's life.
I didn't find the navigating in Le Harve particular taxing (with a map), but I do recall the route I took was quite a climb out (in the pouring rain) and may not be the best way to start a family trip. Especially the rain. Looking at the map, I think it might be worth heading inland for a couple of miles from the ferry port to the suburb of Harfleur, and then follow a valley north to Montivilliers, Épouville, Notre-dame-du-bec, Criquetot -L'Esneval and then head to the coast. It's the route followed by the railway which is always a good sign. It also misses the massive French nuclear installation on the coast which you may or may not want to do.
I have also used the roads through the industrial port area (south of the ferry port) as a route east, especially when heading to the Pont de Normandie. (Ave Lucien Corbeaux, Ave 16 Port, etc). I remember these as being wide and relatively quiet although passing through some post apocalyptic industrial scenery. Admittedly this misses the heart of Le Harve, and there is plenty to see there, but there's always time to sweep downhill into town on your return and see the sights then.