I'm thinking of making a coworker return a "small" amount of money she took without asking
Why do you want to make her return it? What will happen if she doesn't?
I will tell on her if she doesn't do it.
You have every right to ask for the money back. Do this in a way that is non combative and will not get you in trouble at work or with the law. Handling this the wrong way could cost you your job which is not worth doing for such a small amount of money.
Well I'll just take it out of future tips.
If she stole something it is good to get it back, it does not give you the right to steal as well. She might get away with it and then you get blamed for the theft.