As others have said, children under three (you might see it worded as "two years of age and younger") do not need a ticket to enter the parks. They also do not need a FP (if applicable) for attractions and shows (mostly because they won't have a ticket to get the FP in the first place). As others have said, if your child looks older than three, the CM at the gate might ask their age. (On the flip side, if you are making dining reservation, make sure to include them because they do count towards table sizing and maximum capacity counts for the restaurant.)