sand toy vs plaything

sand toy and plaything both are nouns.

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

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

sand toy: Any toy intended to be used on a beach or in a sand pit to play with sand.
plaything: an artifact designed to be played with

Other hypernyms of sand toy include toy.

sand toy (noun)	plaything (noun)
Any toy intended to be used on a beach or in a sand pit to play with sand.	an artifact designed to be played with