Merlin may not have known her name at the time. Rumpel knew the savior would break the curse, but even he had to ask Snow and Charming what the babies name would be.
Merlin knew Emma’s name at the movie theater, but Emma had already been born and several years old, so he could have just learned at.
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"I could have the giant duck as my steed!" --Daniel Radcliffe
Keeper Of Tamara's Taser , Jafar's Staff, Kitsis’s Glasses , Ariel’s Tail, Dopey's Hat , Peter Pan’s Shadow, Outfit, & Pied Cloak,Red Queen's Castle, White Rabbit's Power To World Hop, Zelena's BroomStick, & ALL MAGIC