Young child handling adult perfectly! (2,1,1)
I believe the answer is:
to a t
'perfectly' is the definition.
(I've seen this before)
'young child handling adult' is the subsidiary indication.
'young child' becomes 'tot' (I've seen this before).
'handling' indicates putting letters inside.
'adult' becomes 'a'.
'tot' placed around 'a' is 'toat'.
(Other definitions for to a t that I've seen before include "In every detail" , "perfectly well" , "Exactly right" .)