While we wanted to be subtle, this was almost too subtle - she wore the costume all day, and hardly anyone noticed! But when they finally detected a problem, the responses were excellent.
This instructable documents the creation and care of a 3-armed baby. If you make your own, please post pictures!
Step 1: Find radiation sources
Caution: radiation-induced mutant powers are highly idiosyncratic. We cannot guarantee your child will be born with super strength or telekinesis, or that any extra limbs will be fully functional and/or not evil.
Since we had to leave our real badges at work (pesky government regulations!) Eric quickly cut duplicates on the Epilog laser cutter.
*Hat tip to Stephen King's Dark Tower.
Step 2: Find duplicate outfits
Step 3: Find suitably-sized doll
This doll has realistic plastic hands, and a cloth body - perfect for our purposes. Choose the hand with the most realistic positioning.
Step 4: Prepare doll arm
Stuff a bit more cotton in the upper arm/shoulder region, then hand or machine stitch the openings shut.
Dispose of doll carcass, saving any useful bits for future projects.