Regina said to Hansel and Gretel something along the lines of “You’re not the first boy and girl I sent into that sticky sweet house but you are the first to come out”
Whilst I agree that someone may have put the idea in Regina’s head about the diamond, it’s also possible that Regina thought of it herself as a back up last resort. She had to go to Rumple to tell her how to cast the curse, and whilst there he told her about Emma. Though it did seem like she cast the curse immediately afterwards.
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All magic comes with a price!
Keeper of Felix