As part of the fee increase to $120 Global Entry is now free for children if a parent/guardian is already a member or if they are applying. Global Entry defines a child as anybody under the age of 18. For some reason if you’re applying for a child that was born in 2024 there is a small bug that requires you to change their birth year before adding the parent/guardian and then going back and changing the birth year back.
Hat tip to reddit user stallbackbottle