Each funny face OK for child's play (1,5,2,4)
I believe the answer is:
a piece of cake
'child's play' is the definition.
(I know this)
'each funny face ok' is the wordplay.
'each' becomes 'apiece' (synonyms).
'funny' indicates anagramming the letters.
'face'+'ok'='faceok'
'faceok' is an anagram of 'ofcake'.
'apiece'+'ofcake'='A PIECE OF CAKE'
'for' is the link.
(Other definitions for a piece of cake that I've seen before include "What one may eat at a birthday party" , "... easy thing to do" , "Not much to eat" , "Madeira, perhaps" , "A walk in the park" .)