It does say that there were some family members on the beach who were looking for them. But they were half a mile away. I would imagine that the Mum would have asked them to keep an eye on them while she went to the shops.
Talk about having a go at the poor mum! She left the children with a responsible adult or even several of them and its not HER fault that they wandered off. Lots of people take their relatives along on day trips so they can have some help with the childcare and get to have a little bit of enjoyment themselves.
it is quite unbelievable, and slightly peculiar, isn't it?
We don't even know WHAT she was shopping for, quite possibly food or something. She quite possibly made the quite reasonable decision that her kids would prefer to stay on the beach than be dragged round the shops, and so left them in the care of relatives.