Could just use it as an opportunity to finish off the Dark Fairies once and for all. Sacrificing my son for the greater good would be considered a necessary risk, especially since the blame would be placed on you and your kingdom for cursing my son to begin with.

[Said in a very nonchalant tone. But obviously if he was going to do that, he wouldn't have worked so hard to protect his son from the start.]

That's what will happen if you fail in altering the curse.

[In other words, he won't do as he said now, and Carabosse has one chance. Which itself is a huge risk of its own, but perhaps it could be interpreted that Oberon does have a level of trust in her still, especially in light of the recent "reveals." He's still cautious of course, hence the ultimatum, but there are many potential layers of meanings here.]