A few things to keep in mind:
There is absolutely no reason, by the text, that the actors cast for Georgia and Shaun need to be white. George is described as "pale" after she's been inside for a long period of time, but that's it. The only characters that really need to be cast Caucasian are Buffy and the older Masons (Stacy and Michael). Buffy because she's blonde, the Masons because they're a very specific sort of suburban archetype gone horribly wrong.
Your Shaun and Georgia should be similar in age, because they're going to be playing off each other a lot.
Explaining your casting/supplying pictures is a big, big plus.
At the end of the week—well, Thursday; I'm on a plane all day Friday—I'll pick the dream cast I like best and send them a signed copy of Blackout. Because I hate the post office but hate thinking about addresses more, this is open to everyone.