Young child’s to do addition (3)
I believe the answer is:
tot
I believe this clue is a double definition.
'young child's' is the first definition.
(tot can mean a baby or toddler)
'do addition' is the second definition.
(I've seen this in another clue)
'to' is the link.
(Other definitions for tot that I've seen before include "Nip" , "Aggregate for infant" , "Add up for baby" , "Small measure for a small child" , "quick drink" .)