"Hey suzy can we talk. I never had to have this discussion before, because you have been extremely professional in all you do like when XYZ(point out some good things she has done!)" I think that is why I was a bit suprised when you came over to watch the kids yesterday in a see-through shirt. It was cute and a great beach or work out attire, but it was too much for work. Even work with kids. Moving forward, could you just keep the clothes work-appropriate? If you have any concerns or questions, let me know, but my rule of thumb is if I am wondering if it is ok, it probably isn't."
Easy, peasy. Sounds like a first job OOPS! teach, model and show respect. She is doing a big job for a young woman.