You can travel with one infant in your lap without a purchasing a ticket if:
The infant is less than two years old, AND You are at least 18 years old or the infant's legal guardian, AND Your travel is within the U.S.
When a ticket Is required
You'll need to purchase a ticket for your child when you:
Have a child that is age two or older. Have a child that turns age two during a trip; a reserved seat and ticket are required for the entire journey. Prefer the child to sit in a seat in order to travel in an approved restraint. Have a second child, regardless of age, and you already have a child who will be sitting in your lap. Want your child to earn miles in his SkyMiles account. Will be traveling between countries regardless of whether or not the child occupies a seat."
This depends on whether the child will travel on an adult's lap or will have a restraint with the safety requirements required. This will be charged about 10% of the adult fair. If over 2 years, a ticket will have to be purchased. Contact the airline customer care to know their policies as well as their current charges in your case.