In Peter Pan (1953) Wendy's family seems to be a British family living in London. Why is there a French flag? I would have understood it if there were other flags like the UK flag, but there seems to be only the French flag.
It may be a reference to the Llewellyn Davies Boys who J.M. Barrie fostered. Their birth grandfather was the French-born cartoonist and writer George du Maurier.
Of course it could simply be a souvenir since the Darling's are upper class and are likely to have had the luxury of holidays across the English Channel.