Two head out with toddler sound asleep (4,2,3,5)
I believe the answer is:
dead to the world
'sound asleep' is the definition.
(I've seen this before)
'two head out with toddler' is the wordplay.
'out with' is an anagram indicator (out can mean wrong or inaccurate).
'two'+'head'='twohead'
'twohead' with its letters anagrammed with 'toddler' can make 'DEAD TO THE WORLD'.
(Other definitions for dead to the world that I've seen before include "Sound asleep; unconscious" , "Fast asleep" , "Sleeping" .)