Infant ticket for kids under 2yrs old, no seat provided, has toys, only a basket in front of your seat for your kids but it has weight limit, infant meal provided -- rice cereal, baby food.....
Child ticket for kids under 12 yrs old, has own seat, can choose child meal, has toys
The price of infant ticket is about 10% of adults in the normal rate which is shown on the ticket. The price of child ticket is about 70% of adults in discount rate/ group ticket rate. Both tickets need to pay the surcharge (fuel surcharge, airport tax.....).
For example, the air ticket from Hong Kong to Chiang Mai, Thailand HK$3001 for adults, HK$2361 for Kids, HK$991 for infant (all inclusive surcharge and tax)by Thai Airway.