"Auntie Bea's Day Out"
is a short story by Diana Wynne Jones
Aunt Bea drags Nancy, Debbie and Simon on a day trip to the seaside - with a large bundle of "necessary items" all neatly labelled in case of loss. She insists on picnicking on Island Island, ignoring all the warning signs.
The island shudders and all of a sudden they aren't at the beach any more - they are in the middle of a gun-range. Then they are on an iceberg. The island shudders again...
It is in several short story collections: Warlock At The Wheel: And Other Stories and Unexpected Magic, The Cat Flap and the Apple Pie and The Young Winter's Tales.