
-> used magic to create child
-> magic is disappearing
-> WILL THE CHILD DISAPPEAR TOO??? OMFG–

i think you're partially right and partially wrong with this: anything like the poison has already passed through his system and been eliminated, however it did say that the blessing would be going away so poison would work in the future.
However, we do not know what the princess's status is as a magical being, meaning there is room for doubt on both sides of our argument.
you had to rent p*rn.