the only thing I can add to this conversation is, if you don't feel comfortable raising the issue with the mother, maybe don't invite this child over for a swim anymore. I mean, you can't keep offering her shorts to wear, but you can't feel uncomfortable in your own home either. Personally it wouldn't bother me to the extent where I felt I needed to change what she was wearing, but I don't have teenage boys in the house that I need to consider either.