Ah Jon, this reminds me of a trip I organised decades ago, with my class of eight year-olds, to Holkham. I let them run mad on that huge beach (the sea was way, way out in the distance) while the coach driver and I had our sandwiches. Suddenly to my utter horror, the tide had turned and what seemed like a tsunami rolled up the beach, engulfing my pupils.
The driver and I had several terrified children hanging on to our clothes, and we swam/waded back to safety.
Never went there again!