Young child out of bed? Put two and two together! (3,2)
I believe the answer is:
tot up
'put two and two together' is the definition.
'young child out of bed?' is the wordplay.
'young child' becomes 'tot' (I've seen this before).
'out of bed?' becomes 'up' ('up' can be a synonym of 'out of bed').
'tot'+'up'='TOT UP'
(Other definitions for tot up that I've seen before include "Tally" , "Reckon" , "Add together (driving offences)" , "Summate" , "Determine the sum of" .)