They weren’t casting the Curse just to ‘save their unborn child’; they were casting the Curse in order to get back to the one person who could stop someone who was threatening their entire existence and the existence of every other person in the Enchanted Forest, including their unborn child.
The thing that would have set this Curse apart from the other instances in which we have seen it cast is that its sole purpose was to get them back to the ‘Land without Magic’ so that they could find Emma and stop Zelena from hurting them, their child, and everyone else; had Zelena not intervened and ‘mucked with the formula’, it appears as if the only price that would’ve been paid as a result of casting the Curse was Charming’s death, which is a significant price in and of itself.
As far as the writers’ tossing out their established rules’, I really fail to see how they did anything of the kind, particularly given that this is the first time we have ever seen the Curse cast for largely unselfish and benign motives. Sure, it was a ‘last resort’ option, but is not all that different from a police officer using deadly force to stop a violent situation and save lives.