Hi Hollypop1
You can cross from Santander to the Med in a day. I did it several times (from Bibao) in the past with the children before they grew up!
There are some excellent sites at Cambrils - Torre del Sole, Cambrils Park, Camping Joan, Playa Montroig. The first three are in either the Camping Cheques or ACCI books if you are not going in peak season and are fantastic value for 15 euros. In peak season they are expensive, but I'm sure the same applies everywhere. They all have excellent facilities for children (Joan less so) and I have been to all except Cambrils Park in the last three years.
If you are travelling down through France, the Costa Brava is obviously nearer and I am sure there are some excellent sites there too, but my experience of them is too old to recommend any - I'm sure somebody else will help