Found on the northern tip of the coast of Fajardo, follow the signs to Balneario Siete Mares.
We were fortunate to be staying within a couple of miles of Seven Seas beach, so we found it easily. We arrived on a day the lot was not open, but we found street parking right near the entrance and were able to walk in easily. The beach itself was kept clean, but there was a lot of garbage overflowing the trash cans and a number of stray dogs wandering around, which is common in Puerto Rico. That said, the beach area itself was kept quite clean, with smooth white sand and mellow waves that would be fine for supervising very young children. You have a beautiful view of Cabezas de San Juan and can see where the kayak trips enter the bioluminescent mangroves a ways out. We played in the water and snorkeled here for awhile, then explored the trail through the mangroves to a couple of other beaches.
A note: you can easily reach beautiful Playa Escondida/Hidden Beach from here, but do not attempt to swim there as the current is swift and very dangerous.
There is just one local restaurant that is in easy walking distance from the beach, which is a bit pricey; so, you may want to pack your own food and drinks for the day.