At the same time - trendy, consisting of one child (2,6)
I believe the answer is:
in unison
'at the same time' is the definition.
(synonyms)
'trendy consisting of one child' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'consisting of' becomes 'uni' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one child' becomes 'son' (male child).
'in'+'uni'+'son'='IN UNISON'
Can you help me to learn more?
(Other definitions for in unison that I've seen before include "Simultaneously" , "All together at the same pitch" , "All singing the same notes" , "All speaking or singing at the same time" , "All speaking or singing as one" .)