Here's the xONEx rules in relation to children:
19. CHILDREN DISCOUNTSUnless otherwise specified
Accompanied child 2 -11 - charge 75 percent of the fare.
Ticketing code - base fare code plus CH
or - Infant: under 2 without a seat – charge 10 percent of fare.
Ticketing code - base fare code plus IN
NOTE – If an infant reaches two years of age after travel has commenced but before travel is complete, a full child fare ticket must be purchased for the entire journey.
or - Note
Unaccompanied children are not accepted for transportation using the oneworld Explorer Fare.