When plastic is exposed to light or oils for an extended period of time, a chemical reaction occurs that causes it to turn yellow. Not to worry --- most yellowed-toys can be restored to their original colour using bleach and a few items you likely have lying around your house.
Submerge the yellowed item in a bucket filled with equal parts of water and bleach.
Wait an hour or longer until the bleach has removed the yellow colour. You don't need to do anything else here; just let the toy soak.
Remove the toy from the bucket.
Wash the bleach from the toy using dish detergent.
Dry the toy with a dish towel.
Gently rub superfine sandpaper along any areas that remain yellow.
Wipe the toy with a soft cloth to remove any dust from the sanding.
As an alternative to using bleach, you can scrub the toy with a powdered scouring disinfectant and a moistened scrubbing sponge.