I recommend visiting Kamogawa, Chiba. The station name is Awa-Kamogawa and it's about 2 hours from Tokyo on JR. Nice family safe beaches a short walk from the hotels or traditional Japanese ryokans. If you're interested in surfing, the surfers are there from daybreak to sunset. Surfing rentals are available as well. For a family event, Kamogawa Sea World is also located there and well worth the visit.