Children on team? It’s not the main problem (4,5)
I believe the answer is:
side issue
'it's not the main problem' is the definition.
(a side issue is a secondary problem)
'children on team?' is the wordplay.
'children' becomes 'issue' (issue can mean someone's offspring).
'on' says to put letters next to each other.
'team?' becomes 'side' (both can mean a sports team).
'issue' after 'side' is 'SIDE-ISSUE'.
(Other definitions for side issue that I've seen before include "Illegitimate children" , "It's not an important matter" , "not a major problem" , "minor affair" , "Ancillary consideration" .)