Children's game is quiet before end of party (1-3)
I believe the answer is:
i-spy
'children's game' is the definition.
(I've seen this before)
'is quiet before end of party' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'before' says to put letters next to each other.
'end of' indicates one should take the final letters.
The last letter of 'party' is 'y'.
'is'+'p'+'y'='I-SPY'
(Other definitions for i-spy that I've seen before include ". ... with my little eye (children's game)" , "Wordgame" , "Children's alphabet game" , "Game played with young children" , "Child's guessing game" .)