I can remember it happening several times before, sadly.
I am not usually a 'worrier' at all, but it seems logical to me that sand caves in, and kids are SO keen to dig and dig.
Sadly I think lots of people walk into unforeseen risks in places they don't go all the time. A few years ago, two poor little children were washed away off the Norfolk coast. The parents were convinced that they had been abducted because they were only gone a couple of minutes, and the channel was only knee-deep. But those tidal channels can easily knock a child over when knee deep. I was there when they found the bodies a month later
Poor, poor little Abbie, it must have all seemed such innocent joy to them all, and then....
Unbearable.