from TG website (maybe also in this case: consistently inconsistent!)
Is there a Child's Fare?
For an Infant (up to 2 years of age), the fare for international and domestic flights is 10% of the full fare; the infant cannot occupy a seat. For a Child (2-12 years of age), the fare for a seat on a domestic flight is 50% of the full fare. The Child fare for international flights is 50% or 67% of the full fare, depending on the country of origin and destination.
What if my child must travel alone?
THAI aids Unaccompanied Minors (UM) between 5 and 11 years of age, provided they comply with our rules. The parent must go to THAI office in person to complete a form providing details of the parent or guardian who will see the child off and will meet the child on arrival. When the child arrives at check-in, ground staff will help him/her complete formalities. They will accompany the child to the aircraft and introduce him/her to the cabin attendants who will care for the child during the flight. At the destination, ground staff will escort the child to meet the parent or guardian designated on the form.
Are there special children's meals?
THAI serves special child and infant meals. Requests must be made at least 48 hours before departure. You cannot request specific brands of baby meals or powder milk.
Are there onboard facilities for infants?
Request them in advance. There are free baby bassinets for infants weighing not more than 10 kgs. They can be attached to bulkhead anchors. Parents can use their own baby car seat provided it has been certified by the government of manufacturer's country and provided the infant is flying on a child's fare ticket. Diapers are available onboard and there are fold-down diaper changing shelves in the toilets.