This is why Google was invented. A quick search revealed that every island capable of hosting a permanent human settlement had a permanent human settlement.
But what exactly constitutes a "permanent" settlement?
The answer proved to be 30, perhaps as few as two dozen, but then things got precarious.
You see, a permanent settlement is one capable of producing a stable self-perpetuating population: There are enough adult females producing enough children to replace those who die from old age, disease, or misadventure.
30 seems to be the golden number; any fewer than that and one run of bad luck wipes out your chances of staying abreast of the death rate. Your population can't reproduce fast enough, the old soon out number the young, and eventually you die out or get absorbed by a larger band.
There are numerous islands in the Pacific too small to support a village that could support a smaller band, say ten members or less.
Slowly, the island itself started to come into view.
Bidney Island (I named it after my aunt who gave me the Swiss Family Robinson book) needed to be small and isolated. It couldn't be part of a larger chain or archipelago or else natives might drop by on occasion.
No problem, there are lots of small atolls, reefs, and islands in the Pacific that the South Seas Islanders use as fishing camps, staying a few weeks or a few months then returning to their permanent villages.
Such an island could support six to ten people indefinitely. And with two vast fleets roaming the Pacific in search on one another, there was precious little incentive for any long range fishing trips by the Polynesians.
To work it had be far enough away from Australia to not be part of their network of coast watchers (civilians and military personnel who stayed hidden but scanned the seas for signs of Japanese ship movements; see Father Goose, yet another schoolgirls-shipwrecked-on-a-desert-island story only with the added attraction of Cary Grant and Leslie Caron).
But that would mean the island could be a potential target for either side, which would bring the story to a screeching halt whoever found the girls.
So Bidney Island had to have no military value. That required a small, shallow lagoon, too tiny for large ships to harbor in. It couldn't be a flat atoll but needed a big volcanic cone right in the middle of it, making it useless as an airfield.
That wouldn't keep the combatants away forever, but it would make Bidney Island a very low priority for both sides.
(to be continued)