I'm really sorry to hear this kind of story because it gives kids a taste of "real life" that we would probably prefer they not have to cope with just yet. The suggestion about the home owners insurance is good, that should cover the loss, though it might be just as well to buy new bikes once than pay higher premiums for years...
Your daughter should ask around among the neighbors. It may be that one will have seen something. Perhaps one will have noticed their kid has a new bike.... This is almost certainly the work of a kid (or kids). Going forward, it might be good to scribe something on the frame, address, name, etc., that will help police identify the bike. A Dremmil (sp?) tool should do the trick, and many police departments will mark things free of charge.
It is sad, but a sign of the times. I say it to my wife over and over, "Close the garage door!"
Jim