I would forget about the idea of sanding with the router, for several reasons.
Even if you could find a sander less than 1/4", something that small wouldn't last long at all.
There's also the issue of the drum rubbing on the spoilboard. You wouldn't be able to get to the bottom of the part.
And there's also the issue of rpm. For sanding, I'd think that you'd want to stay below 1000 rpm? Maybe 2000 rpm max, depending on the diameter of the drum.
I've seen sanding heads for large industrial routers, but they were several inches in diameter, and were used on machines where the worpiece was supported with vacuum pods above the table.
You'd probably be better off using a spindle sander and hand sanding them while the CNC was cutting the next batch.