If one of the passengers in your reservation is a lap child (under two years of age), select the option "infant" at the start of purchasing, before clicking on "search and purchase." Lap children fly for free, as long as they travel on the lap of a responsible adult.
See what the rules are for unaccompanied minors*:
Available on the online purchasing, Call Center and at the airport.
No problem, you can include more pounds at the airport during check-in. But attention, if the weight of checked baggage exceeds 23kg will be charged the excess of luggage.