I don't think that true love's kiss between mother-father/son-daughter works on every case. I think that it's a special thing for Emma as she is "the chosen" or whatever and Henry is the only one she loves right now. If it were really for everyone there would be too much curse-breaking kisses available and that would make curses look kind of dumb. There's a lot of limitation on "true love's kiss" that we learn on every episode and I think this is one of them. On condition that the kisses lose effectivity the curse allows Emma's kisses to be very very powerful. Like with Graham and now Henry. The curse is very powerful after all. Or it's supposed to be. I don't know. It's just my opinion.
I agree with the blue fairy thing too. It would be silly for her to miss a chance of eliminating the Dark One curse and even giving up such a rare thing instead. That's another reason I think this mother/son kiss is a one-time thing. Or an Emma thing.
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