Child's play means so simple that anyone could manage it. It was child's play, mere child's play, child's play by comparison.
It is nearly always used by comparison, since something is child's play beside something harder. The phrase is old and started as a literal description of what children do.
1phrasechild's playSo simple that anyone could manage it.
“After years of driving in London, parking in a small village was child's play.”
child's play is an ease.
Child plus play, first used literally of the games children play.
Proto-Indo-European
*ǵelt- — womb
Proto-Germanic
*kelþaz — womb; fetus
Proto-West Germanic
*kilþ
Old English
ċild
Middle English
child
English
child
English, today
child's play
From child + -’s + play, originally referring literally to play by a child. child comes from Middle English child, from Old English ċild, from Proto-West Germanic kilþ, kelþ, from Proto-Germanic kelþaz (“womb; fetus”), from Proto-Indo-European ǵelt- (“womb”), perhaps from Proto-Indo-European gel- (“to ball up, amass”). Cognate with Danish kuld (“brood, litter”), Swedish kull (“brood, litter”), Icelandic kelta, kjalta (“lap”), Gothic 𐌺𐌹𐌻𐌸𐌴𐌹 (“womb”), Sanskrit जर्त, जर्तु (“vulva”).
Etymology from Wiktionary, CC BY-SA