Could be tough. You might try a little bit of rubbing alcohol with a cotton swab. If the sticker is printed under the plastic surface, you should be okay, if it is printed on the plastic, you could take it off. Work carefully and test it!
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This worked for me....
Write over the mark on the toy with a dry erasible marker (used for whiteboards) - mine are called Dry-Wipe pens.
Then wipe it off immediately. The solvent in the whiteboard pen dissolves a bit of the non-erasible marker ink. Repeat until the mark is removed.
Try this on a small piece to check for disasters before attacking the whole toy.
It also works well to remove the marker ink if an idiot uses permanent pens on a whiteboard.