Erich Kastner also wrote "The Parent Trap."
Btw, I somehow slipped up regarding John Peterson (1924-2002) - he was
FAR better known for his series about...
"The Littles."
https://en.wikipedia.org/wiki/The_Littles
He wrote 16 books in the series. It was turned into an animated series
in the 1980s.
(And given that the first book was published in 1967, it's a little
surprising how brave Lucy is, being a younger sister and all.)
He also wrote "The Secret Hide-Out" in 1965. There was a sequel.
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