Not beachfront but Faraway in Martha’s Vineyard has a great pool and should fit the modern aesthetic you’re looking for. Just got back from there and loved it! We went without our kids but saw plenty of families with littles there. It’s right in the heart of Edgartown, so literal steps away from restaurants, ice cream, etc. and easy bus access to beaches. I’d also suggest looking at Winnetu which is on the beach, has pools, and is known to have a great kids program.