Can I book a flight for a baby or a young child through a flight booking API?

Yes, you can book babies and children through a flight booking API, as long as you send their real dates of birth with the booking.

Airlines price young travellers differently from adults. A baby who sits on a parent's lap is treated as an infant and pays only a small share of the adult fare, while an older child usually gets a seat of their own at a child fare when the airline offers one. The API works out which of the two applies from the date of birth you enter, so getting that date right matters more than it might seem. If it is wrong or left out, the search can come back with a full adult fare, or with no fare at all.

Some rules come from the airline rather than from your software. Most airlines allow only one lap infant for each adult travelling, many will not carry a baby under a certain age, and some do not sell child fares on their cheapest tickets, so the child simply pays the adult price. Airlines also have their own way of handling a child who has a birthday during the trip and moves from one age group to the next. Your software passes these rules back to you exactly as the airline gives them, but it cannot bend them.

Before you quote a family, put the exact dates of birth into the search and look carefully at what comes back for each traveller, and check the airline's own rules on children flying without an adult if that applies to your booking.