Hi there, Heather! Welcome to the Disney Parks Moms Panel!
It looks like you'll be visiting the Walt Disney World Resort again soon. Yay! I know you are excited.
You are correct that
kiddos ages 2-years-old and under are considered "Guests of Mickey" and do not need a
ticket to enter the theme parks. In all the years that I have traveled to the Walt Disney World Resort with kids of that age, I have not been asked to provide proof that they are under 3-years-old. I will say that my kids are not tall for their age, so they don't get mistaken for being older than they are.
If you seem to be running into an issue with Cast Members thinking your nephew is older than 2-years-old, perhaps you should take along a
copy (not the original) of his birth certificate to verify his age. I haven't had to provide proof before, but typically I have always had a copy of their birth certificates with me because we needed it for our plane ride anyway. It is always a good idea to be prepared!
I hope this helps all go smoothly for your upcoming trip! Please visit us again with any other questions you may have!
Tabitha