Surprised this story hasn't already been discussed as its been covered a lot on our North West news.
news.bbc.co.uk/1/hi/england/merseyside/8075949.stm
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.