How come when your co-workers get their kids hired just for the busy holiday season they're the rudest most disrespectful brats you'd ever meet?

Entitlement. They most likely have not been through the process of actually going out and getting a job on their own, and therefore have no understanding, appreciation or respect for what that involves. They also may feel there is no way they will be fired.

Because they didn't earn the job or probably don't even want it

Does not happen where i work.