while I cannot give specific advice about your route, friends of ours once biked with a toddler in Germany and as you most likely know from your guide, there are many bike trails that follow rivers and would be very nice for a family trip (not in traffic, relatively flat etc)

As parents, we know that the key to an enjoyable family thing like this involving the physical aspect, is that it is fun for everyone. As you say, part of that will be the planning (safe road, bike paths etc) but also a big part as others have stated is that you slowly get the family out biking. Doesnt really matter on what bikes at this point, but with short fun bike rides (to the ice cream store or whatever) and building up in time on bike--and ALWAYS have snacks, drinks.