Inclusive event about to include games for children (4)
I believe the answer is:
open
'inclusive event' is the definition.
(an open is a sporting contest open to amateurs)
'about to include games for children' is the wordplay.
'about' becomes 'on'.
'to include' means one lot of letters goes inside another.
'games for children' becomes 'pe' (short for Physical Education, a school subject).
'on' placed around 'pe' is 'OPEN'.
(Other definitions for open that I've seen before include "begin to > crack" , "Frank - accessible" , "Not closed or secret" , "Overt" , "golf contest" .)