Hmmm...the kid's on to me

Yesterday I was trying to speak Chinese with my wife and stepson, who had just gotten a barrage of immunizations all at one go and were complaining that their arms hurt and they felt tired. I wanted to warn them, "You guys might get a fever," which would be something like, "Kě​néng​ fā​shāo​." Unfortunately I got kě​néng​ confused with kě​yǐ​, which is more along the lines of "you can" or "you may" rather than "maybe," and so to Helen's amusement I said, "Kě​yǐ​ fā​shāo​." I knew as soon as I said it that something didn't sound right, and she straightened me out and all was well.

But then sure enough the next day they were both running fevers, and Helen was taken aback when suddenly, about mid-afternoon, Kai announced, "This is all Daddy Kenny's fault."

HELEN: How is this Daddy Kenny's fault?

KAI [grinning]: It's his curse.

HELEN: His what????

KAI: Well, he said, "May you catch fever..."