I have just looked at the Danube, its a little too central Europe, A lot of driving and couped up kids. Im sure someone will recommend a means of public transport and if it was a case of bikes and panniers then i would consider but its the 2 trailers in tow also. We need to do a shakedown and indeed buy some gear for the trip yet so the cargo trailer may be surplus to requirement but I'm almost certain that we will need it for the camping gear at least.
We are not hill averse but I am getting risk averse in my old age(34, i used to be a real hair-raiser, no handers and all sorts
so i would take a reasonably hilly track than a flat road. The 8 year old knows the benefit of a hill to his training and the 4 year old just puts his feet up. As long as its not repeatedly undulating we should be fine.
Has anyone don't the stretch of EV1 from from Hendaye northwards, I know its not camino but we are among well travelled people.