Two ideas occur to me:
- Have the kids buy you a new one, they can keep the brick as an object lesson.
- Contact the seller, explain what happened, and ask for an exchange.
The Kindles have no way to recover from a partially installed update.
They do not have an 'undo' or 'do over' function in the OTA updater.
It could be recovered using the serial port, but that means taking it apart and doing a lot of skilled labor.
Most people would just exchange it (and maybe throw a kid or two into the deal).