Nothing really happens to the phosphors if the glow in the dark toys are just sitting in the dark. They don’t degrade simply by being stored without light. The phosphor molecules are stable at room temperature. The only thing is (since they’re not being “charged” by light) they won’t glow when you pull them out. But once you expose them again to a light source, they’ll recharge and glow just as before. So no harm done.

You can leave the toy under fluorescent light tbh. Over years of repeated exposure, though, there can be some gradual degradation. UV light in particular can slowly break down the crystal structure or the polymer that binds the phosphor to the toy. But for a plush kept indoors under a fluorescent bulb, it’s almost a negligible process. It will take years before you’d notice dimmer glow.