What kids might do when it’s raining outside to become active (4,4,4)
I believe the answer is:
come into play
'what kids might do when it's raining' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'outside to become active' is the wordplay.
'outside' is an insertion indicator.
'become' becomes 'come' (synonyms).
'active' becomes 'inplay' (I've seen this before).
'come'+'inplay'='comeinplay'
'to' inserted within 'comeinplay' is 'COME INTO PLAY'.
Can you help me to learn more?
(Other definitions for come into play that I've seen before include "no longer sit on the sidelines" , "Arrive as important factors (into a situation)" , "start to feature?" , "When it's wet outside kids may" .)