Two of the main child characters were Australian. Dana Wilson as Barbie and Colin Peterson as Georgie. Both children have very noticeable accents, Colin Peterson's is particularly strong but it's explained by the fact that his mother an actress had spent time in Australia. Dana Wilson's character however was meant to be from the Kings Cross area of London and sister to two boys who do have London accents.
The song "Gotta Have Rain" made a brief appearance in the British record charts. It was also a big US hit for Eydie Gormé.
Kathleen Harrison was actually 66 when the film was made, her son played by very good, Sean Barrett supposedly about to be 15, in the age children could leave school at the time. (School leaving age did not rise to 16 until 1972 and incrementally raised by 2015 to 18). Sean Barrett does look older than 15 but he was only 19, not so much older than characters age.
When the children were camping in the woods at night, the two young Australian actors were startled by a bird. Ridiculously the call was that of a Kookaburra, an Australian and New Guinea bird. Nothing naturally sounds like that in the bird population of the U.K.! It's not unusual to be hear birds and other wildlife not native to the supposed setting of films but usually it's a swap of one jungle type environment for another, hoping film goers wouldn't realise.