I can see your point about why Emma would most likely be protected. I don’t know about Henry, he was born in our world where there is no magic and because he’s not pure Snowing, the magic may not be as effective.
Also I kind of don’t want Emma and Henry to be protected by Rumple’s protection spell because it takes away an element of danger for Henry and Emma. Obviously neither of those two is going to be killed off, but the thought that they could be is what’ll keep audiences glued to their seats.