Keep going before second child appears (5,2)
I believe the answer is:
press on
'keep going' is the definition.
(I've seen this before)
'before second child appears' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'second' becomes 's' (common abbreviation).
'child' becomes 'son' (male child).
'appears' says to put letters next to each other (some letters appear after others).
'pre'+'s'+'son'='PRESS ON'
(Other definitions for press on that I've seen before include "Hasten insistently" , "Persevere doggedly, quickly" , "Continue going forward" , "Keep going forward" , "don't stop!" .)