The following is a little bit of thinking aloud....
As Stu is short of time and therefore staying around the north of Spain I am thinking that after several days of looking around France his little one would probably like a day by the sea so am thinking that he comes into Spain via Irun/San Sebastion and in the Santander area locates a campsite (various to choose from- details later but some of the sites close at the end of September). (If Stu comes in via the tunnel there's a bit of duplicated mileage) From Santander there's a short run up to Reinosa and there I can put him on a 70 odd mile trail that will take him to Potes (Picos de Europa) where there is one short - 25 mile- trail over to Sotres. And the cable car up to Fuente De. As far as I'm aware this one of the few trails allowed in a Spanish National Park. Then bring him across towards Villafranca on the day that the Santander ferry gets in. Campsite is at 42.263332,-1.738611. Santander to campsite is 324kms. The following day take in the Bardena Park, Argaedas (25kms).
Then onto Cuenca, or if Stu is short on time I can send him towards the Smugglers Pass and Andorra. The old town of Cuenca is very beautiful and worth a visit with a campsite in the forests nearby.
I've been concentrating on getting Stu's northern Spain route sorted but with the information from Mojo I've had a good look at that and have managed to find a trail that links up with the route he gave us but this one starts in Quadix (the town with the cave houses) and it crosses the Gorafe desert before entering the Cazorla valley. So for the return from Tabernas, the choice is an hour up the A92 m-way or a back road route that will take us over the Puerto de la Ragua (2000 metre high pass) - all paved but demanding.
Is this the sort of trip that you are thinking about? Obviously there's a lot more to input and several days not yet accounted for but rather than me spend too much time on the wrong path, I thought I'd ask opinions.
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Bardenas
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Cuenca
Let me know what you think