sex toy vs plaything

sex toy and plaything both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex toy	Yes	No	No	No
plaything	Yes	No	No	No

As nouns, plaything is a hypernym of sex toy; that is, plaything is a word with a broader meaning than sex toy:

sex toy: A sex toy is an object or device that is primarily used to facilitate human sexual pleasure, such as a dildo or vibrator.
plaything: an artifact designed to be played with

Other hypernyms of sex toy include toy.

sex toy (noun)	plaything (noun)
A sex toy is an object or device that is primarily used to facilitate human sexual pleasure, such as a dildo or vibrator.	an artifact designed to be played with