Sleep so long with son (3,4)
I believe the answer is:
bye-byes
'sleep' is the definition.
'so long with son' is the wordplay.
'so long' becomes 'byebye' ('bye-bye' can be a synonym of 'so long').
'with' means one lot of letters go next to another.
'son' becomes 's'.
'byebye'+'s'='BYE-BYES'
(Other definitions for bye-byes that I've seen before include "Informal repetitive farewells" , "(Child's word for) sleep" , "Sleep for little children" .)