If's it's latex, let it dry completely and you should be able to scrape or pick it off with your fingers/nails. It's happened to me.

Denatured alcohol will dissolve latex paint. Scrap off what you can and let the item soak in some alcohol. Rubbing alcohol will do. Then launder. There might a little staining if it has bright/dark pigments.

I have no help to give, but having done similar things, I FEEL YOUR PAIN. :)

Goof off. That's the name of a product that will remove dried paint. It's available anywhere they sell paint. It's very effective, but you may it may make more than one attempt, if you have alot of paint on your pants. Good luck.

Leave it, gives it character.

If you can't get it off then keep the jeans and make them your "paint" pants.

I agree with HappiLady. Save them and use them the next time you paint.