According to this website the best beach for families in Spain is Playa de los Bateles, situated in Conil de la Frontera (Andalusia, Southern Spain).

To reach this conclusion they have studied several factors such as calm and warm waters, nice clean sands and that the beach is not too crowded.
There are many other beaches in the Mediterranean coast of Spain with the same characteristics, that might as well be the perfect destination for families with children.
Good examples are the beach of Garraf in Sitges, Catalonia, Santa Eulalia Beach in Ibiza and the Playa de la Pineda in Tarragona.