To make things interesting, I'm thinking that all three kids have some degree of power and that the family has attracted some sort magical McNasty that is trying to wipe out the family line. However, I'm having a hard time figuring out which McNasty (my knowledge of world mythologies is hardly encyclopedic), especially that'll make for an interesting and viable opponent for the PCs at their currently power level.
So, suggestions welcome.
Not sure what would fit your campaign best but here are a couple of ideas...
The Vampire Connection - With the Red Court a going concern and still pushing the WC hard,
they want two things - increase their power and decrease the WC's power. Turning children with the power to become wizards accomplishes both goals. They've got the parents out of the way, it's time for a "guardian" to step in...
Caught in Sidhe Politics - If one or more of the children are changelings, they (and their family) have become counters in a covert war waged between two sidhe nobles - one of whom is the father.
My Ancestor's Ghost - One of the children's grandparents wasn't able to handle the power his family had and stay sane. In an effort to end the magic, and his line, he killed one of his sons (an uncle of the children) before being cut down by police. He's still trying to end his progeny...
My initial idea has already been stated, but I'm trying to flesh out the reasoning a little more. In Changes, we've seen that the Red Court uses quite a few Jaguar Warriors, which my interpretation of them appear to be Red Court infectees that are the semi-mortal protectors of the Reds during the day time
. Perhaps the Red King or someone in his court wants to infect the children and then provide them with the Red Court equivalent of the Tattoo of St. Giles (perhaps the Mark of the Jaguar). This would leave them mortal-ish, with access to their burgeoning magical powers and with the right application of diplomacy potentially under the sway of the Red Court.
I'm mulling over other potential ideas right now, but I also like the Warlock who is trying to eat their power. Maybe he performed necromantic curse that ripped the power and the life out the driver. It's possible the Warlock didn't know the entire family was in the car when he did this ritual.