I think we are close to making a decision, we will spend a couple of days in Paris. Whilst I love the idea of slowly winding our way down France, I'm not sure it is best for the kids to travel by train for a couple of hours every day, then settle in a different place (which sounds a little like the camino now I write it!). Thank you to all the ideas put forward, they have given me hours of searching online for different options, and a couple of spreadsheets checking prices with different ideas.
We thought of starting before them, but we are starting and finishing together. I was keen to walk the Voie de la Nive, but was the only one in the family. We anticipate completing the whole camino SJPP to Santiago, however plan on going via Valcarlos. I don't think there will be any other alternative.
Thanks again to all. Only a couple of weeks before we start. Surreal!