I can only really help you with the west coast of France, as I haven´t driven to Valencia via the East coast since I was a pre-teen. I do remember stopping in Andorra which was fun bit little else.
On the West coast, you have two options, to stick to the coast completely, or to go inland a bit.
Drive from Valencia to Bilbao, stopping in Belchite, this place is incredible, it is a hidden gem, you have to arrive at 12.00 as they only open the gates once a day. It is a village destroyed in the spanish civil war and abandoned. Have you seen Pan´s Labyrinth? It was filmed there, it has bombed out cathedrals, one with the bomb still embedded in the tower! Also, I never learned a thing about the Spanish ivil war at school, but it is such important recent history and explains a lot how incredible a country it is to go transition so quickly to democracy etc etc. Incidentally, Valencia was the only place never to fall to Franco's forces which is why it is so modern, it was heavily bombarded by land and sea. Or if history isn't your thing, Zaragoza (half way point, about 4 hours) where you could go to the big shopping mall (Puerto Venezia - I go to Ikea for lunch here!!) or into the city itself. OR, instead, if you carry on another hour, hour and a half, visit the vineyards of La Rioja, the buildings have been designed by famous architects so they are often fun for children as well as adults.
disfrutavitoria.com/wp-content/uploads/2010/10/rioja-03.jpg here is one, sorry i Can't do licky links on my spanish keyboard!
Then towards Bilbao, up via San Sebastian (both these towns have good things to see, Bilbao has the famous Guggenheim museum which you could do in a couple of hours and San Sebastian has the incredible old town and the famous pintxos (tapas). But I would cross the border into France to do your first overnight because of the cheaper accomodation. Biarritz would be a nice plae to stay, it might be too far to get up to Bordeaux. From there, you can stick to the cost, or go inland a bit. I usually go inland because I stop at Oradour sur-glane, I mentioned it before, really moving and worth a visit if your children are old enough/have studied nazi germany at school. It is horrific at the same time. I then overnight in Paris (or the outskirts!) THen calais.
THe other way has Mont St-Michel, I´d really like to visit there one day, and you can get a ferry from Le Havre or maybe even direct to Ireland from there.
I'm sorry I can't help much with the way down. Maybe you could stop in Provence? Avignon for the papal palace? I'm really excited for your holiday now, you're going to have an amazing time!