TG actually charge 25% of the adult fare for infant without seat.
Fare rules for BKK-FRA basis JRUTH departure 14 December as example:
CHILDREN DISCOUNTS C
NN/ACCOMPANIED CHILD PSGR 2-11 - CHARGE 75 PERCENT OF
THE FARE.
TICKETING CODE - BASE FARE CODE PLUS CH.
MUST BE ACCOMPANIED ON ALL FLIGHTS BY ADULT PSGR 12
OR OLDER.
OR - INS/INFANT WITH A SEAT PSGR UNDER 2 - CHARGE 75
PERCENT OF THE FARE.
TICKETING CODE - BASE FARE CODE PLUS CH.
MUST BE ACCOMPANIED ON ALL FLIGHTS BY ADULT PSGR
12 OR OLDER.
OR - INF/INFANT WITHOUT A SEAT PSGR UNDER 2 - CHARGE 25
PERCENT OF THE FARE.
TICKETING CODE - BASE FARE CODE PLUS IN.
MUST BE ACCOMPANIED ON ALL FLIGHTS BY ADULT PSGR
12 OR OLDER.