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.
Step 5: Prepare third sleeve
Test your doll arm inside the sleeve - is it long enough, and properly plump? Did you use the appropriate matching-side sleeve?
Step 6: Sew in third sleeve
Sew sleeve in place, stitching right inside the edge of the split seam. (you'll likely have extra bits of cut sleeve protruding beyond, which is fine.)
Turn the outfit right-side out to admire/check your work.
Now you can sew your third arm into the sleeve, anchoring it at the shoulder only for extra-lifelike flop. Alternatively, go put the suit on your 3-armed baby.
Step 7: Dress your happy mutant
Step 8: Put her in the carseat
Be sure to secure all three arms underneath the shoulder straps.
Step 9: Visit the farmers' market
Most people were completely oblivious. When people smiled at Corvidae, I waved her third arm back at them - everyone smiled again and returned the wave, clearly not bothering to count arms. Only those who had several minutes to examine her up close while I paid for groceries ever noticed something was amiss. You could tell when they saw it - their eyes were suddenly huge, surprised and amused/horrified all at once. Even after we told people she was wearing her Halloween costume, it often took a while for them to catch on.
My favorite anecdote: a lady came up to us at the market, and asked if she could say hello to the baby. We watched with amusement as she spoke to Corvidae, put her finger up for the baby to grab - and offered it to the third hand. She seemed a bit disappointed when the baby didn't grab her finger, and stepped back. Finally her eyes widened in shock. "Your baby has three hands!"