Playa la Malvarrosa, Valencia
The beach goes for miles. Both sea and beach are very clean and even in high season don't feel cramped at all. There's playgrounds all the way along the promenade and the beach itself. There's also cycle lanes, beach volleyball, medical points, showers and of course plenty of cafes and restaurants. And if all that is not enough, there's a giant playground at the end of the beach, in the beginning of the marina. Well connected by public transport. Bus 19 (from city center), 31 and 32. Metro line 5.
Los Viveros Park (Jardines del Real)
One of the oldest parks in the city. Beautifully landscaped, it has a few playgrounds, a miniature cycling city, a Natural Sciences Museum and a cafe. It's connected with the River Turia park and easily accessible.
Calle San Pio V, Valencia 46010 Tel. 96 352 54 78
River Turia Park, Valencia
it's a 9 km long park along the bed of river Turia that was diverted away from city center after the big flood of 1957. You can walk the full length of it, the landscape is varied all the way along and is full of children playgrounds at various points.
You can also cycle or run if you wish. Towards the sea end is the architecturally amazing 'City of Arts and Science'. Just before that is a giant sculpture of Gulliver acting as a huge climbing frame and slide for children.