It's always painful when someone you think you can trust betrays you. You could try a guilt trip on them... that works with some children. Or try hiding your money and instead placing a note to them in the place your money usually is, letting them know you're not going to stand for thievery and that if they want money, they can either ask you for it, or find a way to earn it. Offer them odd jobs and such. And start keeping a close watch on your money, counting it regularly to see if any is missing.

Don't be too upset that they're doing this. Most children go through a thing like that, it likely isn't anything personal.