Every ferry company defines infant & child discounts differently. However, most common discounts apply as below:
Infant: 0 to 4 or 5 years old.
Child: 4 or 5 up to 10 years old.
Infants need a zero priced ticket to travel & usually they don't occupy a seat, while children should pay around 50% of the fare. These discounts are not always available in all seat types. In most cases, these can be booked on an economy seat (deck).