Chip,
How big is the hole now? I'm not real familiar with that particualr bike but I think that if the threaded fixture that the bolt screwed into has falled out...or into the swingarm, you could possibly drill and tap for another bolt. Not sure how much meat is there though.
You could also take the bolt and thread it onto the nut then have a "good" welder weld the nut in place. Just make sure there is sufficient hole behind the nut for your new bolt to slide through.
Chip, try your best to keep a good eye on jr's bike. Kids tend to ride em till the hinges fall out without a second thought about maintenance.
"If they're runnin...ride em!" :)