I don’t think THO had anything to do with it, because then they would have known about Henry for a LONG time before they even first got into Storybrooke, and it seemed to me like they came to SB to destroy magic but after that initial goal found Henry and decided that getting him was more important. I think that either August, in an attempt to get Emma back on the path of breaking the curse, or Mr. Gold, as part of his “procuring” Henry for Regina, convinced her. Of course, both of these characters would have had to go through someone else to talk to Emma or Emma would have recognized them in SB.
Other possibility: Some outside party we’ve yet to meet (or, yes, maybe the Home Office) has been interested in breaking the curse from the beginning. This person convinced August to go break up Emma and Neal and told August that Neal was Baelfire, convinced Emma to give Henry up for adoption and then connected baby Henry to Gold when Gold was looking for a baby for Regina, and gave Mary Margaret the book and suggested that Henry might need it. I’m calling this my Mastermind theory, and I personally think that the outside party might be the Blue Fairy.