Fabric paints are meant to be non-toxic since they frequently come into contact with skin. That said, it might not stick to your lenses very well if they're made of plastic, but health-wise you should be fine. I would say you should experiment with it on a similar material to your lenses. The only concern I would have is chipping paint getting into your eyes, but if the paint actually adheres and is only used on the outside of the lens you should be good.