mumble-the-peg vs child's game

mumble-the-peg and child's game both are nouns.

Word	Noun	Adjective	Verb	Adverb
mumble-the-peg	Yes	No	No	No
child's game	Yes	No	No	No

As nouns, child's game is a hypernym of mumble-the-peg; that is, child's game is a word with a broader meaning than mumble-the-peg:

mumble-the-peg: a game in which players throw or flip a jackknife in various ways so that the knife sticks in the ground
child's game: a game enjoyed by children

mumble-the-peg (noun)	child's game (noun)
a game in which players throw or flip a jackknife in various ways so that the knife sticks in the ground	a game enjoyed by children