One of five children initially offered a substitute for rice? (6)
I believe the answer is:
quinoa
'substitute for rice?' is the definition.
'one of five children initially offered a' is the wordplay.
'one of five children' becomes 'quin' (short for quintuplet).
'initially' indicates taking the first letters.
The first letter of 'offered' is 'o'.
'quin'+'o'+'a'='QUINOA'
(Other definitions for quinoa that I've seen before include "Grain-like crop from South America" , "South American grain" , "Vegan food" .)