Allison!
This is a great question!
So there are two possible answers. Let me give you both, so you can decide what is best for you.
Option 1: If you child will actually be younger than 3 (my youngest was 2 years and 10 months old on our first trip, so on this I can speak with some experience), you simply DO NOT need to buy a ticket. Period. You will need to indicate him/her on your resort reservation, as well as when you make Advanced Dining Reservations for meals, so that they can select appropriate sized tables, but you don't have to get them a ticket. We LOVED that on our first trip!
Option 2: If your child has had their 3rd birthday by the time you will visit, then you buy them a child's ticket, as you would if they were anywhere from 3 to 9 years old. You should see that option when you select your tickets for both adults (10 and older) and children.
When you go in to the website to
purchase tickets, you first select the type of tickets before you indicate how many you need for each age range. So you will see the option to select between the two later in the process.
I hope that this helps?
Have a Magical Vacation!