Sitges (30m south of Barcelona El Prat Airport by train). Coast, nice beaches, lovely long promenade, (lots of people walking, kids on bikes/rollerblades, old people with dogs), little boutique-y shops, nice old town. Gay Capital of Spain (unless Cadiz has taken the crow?) just so you know. Google it, lovely place. i'd go with dc in a flash.
Further down the coast: Tarragona. Roman stuff galore including an amphitheatre overlooking the sea. 1hr/1hr20ish south of Barcelona El Prat by train, or 15 minutes from Reus-Ryanair (hiss, spit). Very quiet, near Port Aventura theme park and aquapark. Lovely people, polite in hotels/cafes etc, very child-friendly. Big 'paseo' leading to the 'balcony' overlooking the sea - you're supposed to walk along it everyday and look at the sea! I live here with DS age 16mo and it's GREAT for small ones. Perhaps not much to do if your dc are teens. barcelona much better but obviously noisy.
We've also been to Santander - gorgeous place, huge beach, great for surfing, kite-surfing. Not sure if it's very safe for swimming, you'd have to check that. Great food, great summer weather. There's a local airport, not sure who flies there.
Have fun!