We're just back from a beach vacation and here is what we did: have kiddo in UV protective wear so you only have to put sunscreen on legs and arms (and face). Have something that gives you shade (big umbrella, beach shelter thingy... whatever works for you). Bring a BIG bucket and squirties.
My daughter spend most of the day at the beach playing in the sand with a bucket of water. To keep her cool I regularly wet her bathing suit, she dipped her feet in the water and I had her stay in the shade. It was between 95 and 100 degrees every day and lots of fun!
Please know that even if they stay under the umbrella sunscreen on all of their exposed skin AND a hat. You can wet down their hat and clothes and the evaporation will help keep them cooler. Also lots of liquids to keep hydrated.