As Bob said: a baby always works
But seriously, you could make the shide in question come of as generous and trustworthy and then have her make a similar bargain as Maeve tried to make with Harry in SK, to get her pregnant. If your players are not of the paranoid kind (which would, in my experience, be a rare breed
), they might even fall for a night of pleasure, when the fae hands them whatever they need, without being pressured into it.
To make things worse, they bring the pregnant fae to a place in the nevernever where time is running faster than in reality. A lot faster. A few weeks later, the characters have a whole bag of new enemies: their full grown changeling kids.