One of five children initially orders all vegan food (6)
I believe the answer is:
quinoa
'vegan food' is the definition.
(I've seen this before)
'one of five children initially orders all' is the wordplay.
'one of five children' becomes 'quin' (short for quintuplet).
'initially' suggests taking the first letters.
The first letters of 'orders all' is 'oa'.
'quin'+'oa'='QUINOA'
(Other definitions for quinoa that I've seen before include "South American grain" , "Grain-like crop from South America" , "Vegan food" .)