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?

See"saw`\, n. [Probably a reduplication of saw, to express the alternate motion to and fro, as in the act of sawing.] 1. A play among children in which they are seated upon the opposite ends of a plank which is balanced in the middle, and move alternately up and down.

Source: Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc

(I'm getting a lot of mileage out of cutting and pasting from the dictionary this morning....)