Made for sanding picture frame miters, Logan Graphic Products F200-2 Precision Sander Elite can sand molding up to 3-1/2″ wide. The 10″ disc sander can be set up to form perfect left-hand or right-hand 45º miters. Out of the box, those are the only angles it can sand, but I bet with a little modification you could extend its abilities.
The Sander Elite comes with squaring and disc cleaning tools. Pricing starts around $130 before shipping. If it wasn’t so expensive, this looks like it would be an excellent “power” tool for kids.