Northern spain

What part of northern Spain--the Basque Country? You can also check Bilbao and even San Sebastian/Donostia, but I think it's unlikely you'd get close to the fare you'll find into Barcelona and Madrid. Maybe you will if you fly out of a small, high-cost US airport where even big-city destinations are expensive.

You can check rail schedules from Barcelona and Madrid on the Renfe website to see whether things look appreciably better from one of those cities. https://www.renfe.com/es/en. Use Barcelona (Todas) and Madrid (Todas) as your points of origin. Use San Sebastian/Donostia if that is your destination.

I'm seeing more train departures from Madrid than from Barcelona and more arrivals in San Sebastian than in Bilbao (though Bilbao is a larger city). There's not a good fast rail line between those two cities. One normally uses the frequent bus service.

It's tricky to fly into Barcelona or Madrid and take a train on to San Sebastian or Bilbao the same day. It's tough to confident about when you'll be in a position to step onto a train in downtown Barcelona or Madrid, so many would suggest that you not buy the train ticket until you're on the ground in Spain.

If you were to replicate our 2013 trip (although t he at was in October), we flew in to Madrid, and had several days there. We then went by train (5 hours) to Bilbao. From there, we rented a car, although you could stick with trains if you wished. We worked our way east, including San Sebastián (Donostia) and Girona, eventually reaching Barcelona for a few days there. We flew home from Barcelona, which avoided backtracking to Madrid, had we flown roundtrip.

Do you suggest we fly in to Madrid or Bilbao if fare is very similar. We will plan to fly back from Barcelona. We will have 2 kids, 6&8 yrs old with us.

If the goal/plan is to be in the north, flying there on the same ticket only makes sense.