Lisbon is about 15 miles from the sea which means that there are plenty of beaches in and around the city to explore. You do not need a car as these beaches are accessible via public transportation. The closest beach to a train station is the Praia do Tamariz which is also kid-friendly. The waters are calm and clean. To get here, take a train from the Cais do Sodre station which departs every 20 minutes. The journey is free if you have a Lisbon Card.