I think since she enacted the curse already then the counter to it was already in play. Kill the child, kill the curse.
Anyway someone on another board mentioned this as something Emma could have been thinking while talking to her parents and I had completely forgot about it.
August told her he came through the wardrobe with her. She didn’t believe at the time but once she believed in the curse I’m sure she would believe that as well.
Emma doesn’t know about the “one person” rule of the wardrobe (at least I don’t think so. She’s skimmed the book but I don’t think she full on read it). So perhaps in Emma’s brain she’s thinking her parents entrusted her fate to a 7 year old boy and then he abandoned her too…