I live in Cornwall & go to the beach a lot.
My DC are 12, 11 & 9 so this is what suits us. Also I drive to beaches which influences what I take.
In the car I leave:
1l Chillys bottle of water. Drinks always get finished on the beach & everyone wants a drink on the way home. Apples/snacks for same reason.
Bag with shorts, t-shirt, underwear & hoodie with our changing towel. You can buy fancy Dry Robe types but ours is years old & is essentially a large towel tube with an elasticated top that you put over you head & get changed under. It's perfect.
My DC put their swimsuits on & wetsuits on up to the waist before we leave home so arrive at the beach in them. We take bodyboards, frisbee, ball & kite for entertainment. Youngest also enjoys a bucket & spade sometimes.
On the beach we take camping chairs & floppy hats OR a beach shelter. I take a couple of hand towels, packed lunch & drinks but if you want less to carry most beaches have cafes/takeaways etc.
Go to lifeguarded beaches, stay within the red & yellow flags/correct areas, use sun cream even if it's overcast. Check the tide times & be careful not to get cut off with an incoming tide.
Mine stay in the water on body boards for hours & love it. They pop in & out for drinks, lunch & games. They get changed back at the car because it's easier than bringing extra stuff on the beach & wetsuits are heavy when wet!
A waterproof case for your phone is ideal so you can take photos whilst actually in the water.
Enjoy!!