From what age does my child need to have a Disney World ticket?
Children aged 0-2 years old do not need to have a ticket to enter any of the parks at Disney World. Disney World tickets
are only required when your child reaches 3 years old. If your child turns 3 when you're on holiday, a ticket will be required for the remaining days of the holiday after he or she turns 3.
People of course want to know how this is "policed". People have shared stories managed to get in up to a 5 year old into the parks without a ticket, but there doesn't seem to be a set rule when it is questioned. You could be asked to show ID for your child to prove they're under 3.
You'll have to ask yourself if trying to risk it is worth it. It probably isn't. It is best to make sure you all members of your party age 3 and up have the correct tickets.