I have been to spain (murica area) for the last 4 years from 20-24 feb to visit my Mum. Without exception I have always managed to go to the beach (water too cold to swim though) and have always come back with a tan. Gets chilly at night though.
It would probably be nice, we've been to spain in low season and, as others have said, mostly nice. Only thing though if you, or DC's really enjoy swimming pool/swimming in sea part of holiday, then you might want to go somewhere hotter. Only say this as we had hols to algarve this year and although lovely to get some sun, it wasn't warm enough for swimming and DS a bit disappointed.