Well, seeing as how Ohio says that a crosswalk is for pedestrians (ORC 4511.01) and as how bicyclists aren't pedestrians (id.), it seems that they shouldn't have been crossing in the crosswalk.
Now, whether that actually would put the kids "at fault," in an accident as described is another matter altogether. I don't know what an Ohio court would say in terms of that. It's likely that in a criminal case, the driver would get off, but that in a civil negligence case, he / she would be liable for damages (very probably reduced because of the kids' actions).
Of course, I can't say for sure on any of these points, as I'm not about to do the research necessary on Ohio court cases, but going with what I remember from the Multistate Bar exam, that should be close to right.