It seems to me that Melanie Safka was the first breakthrough singer with that teeny girl sound. Was there anyone before her.
Then Cindy Lauper was next as far as I can remember.
Today there are a lot of women performers who have that "girly" sound, but was Melanie the first?
There were plenty in the 1950's.
https://www.youtube.com/watch?v=X9Qy9EXaOo0
Melanie was more hippy dippy innocence. But being "hippy" it implied sexual savvy. "Brand New Key" was also banned by some radio stations because it was thought to symbolize sexual intercourse.
I don't think any of the artists in the 50's that you pointed to had that sort of infantile fake innocent little girl sound. It was almost pedophilic in its sound, conflating an infantile girl voice with roller skates a kid's and mostly girls toy, and the suggestive imagery of skates and key.
The only next song closest to this but subsequent was "Girls Just Wanna Have Fun" which most so many guys changed to Girls Just Wanna Have Sex.