Swing and roundabout

"What one loses on the swings one gains on the roundabouts."

In this idiom, are "swing and roundabout" the playthings of children? roundabout = merry-go-round?

Then how did this idiom come about? Why with "swing and roundabout"?

BTW, do you know how did the word "seesaw" come about?
Why is it spelt this way?

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