Having recently done a nearly 4000km journey from our home in Asturias thru France and back home via Girona (nr Barcelona), Aragon, Navarra etc with 3 children aged between 7 yrs and 7 months my advice would be to fill your car full of snacks and picnic lunch stuff! We spent almost 100€ on food on the way to France as we stopped at proper places…
Oh and add about 3 hrs onto your rout planners estimated time to allow for all the stops you need to make!!! Our 6 hr routes were about 8hrs after stops..and that was when we were really evil about the timing of them, it was 10hrs before! Try and plan when to stop..
And depending on the age of your children, little surprise “presents” (cheap things such as new colouring books or reading books). We found a story CD worked really well as we all enjoyed Charlottes Web (if anyone has CD 3…) but don´t rely on DVD players as ours stopped working as we crossed the French border!! We had some good conversations though!
Try Santander or Torrelavega as well for accomodation if you have not already found some..or if you get desperate enough to do a 3.5 hr drive, my apartments free… 😉
Good luck…budget for petrol especially if heading thru France as it is SOO expensive there, Spain is between 30 to 50 cents cheaper a litre!!!
Have a good holiday..