A mix of laundry bleach and borax might work. Or maybe just borax. Leave them in a spot that gets a lot of direct sunlight and they should bleach pretty well(real primitive, lol!). On the other hand, I've never tried the borax. I would guess the bones need to be pretty clean, though. I will throw a couple into some borax mix this afternoon and see if it. Works.
Hawkdancer