On a recent thread someone mentioned General's "Sketch and Wash" pencils. I bought some on Amazon but have not used them yet. The reviews on Amazon indicate they wash out completely, but you might want to test first:
https://www.amazon.com/General-Penci...dp/B002Y2MD16/
They are probably available in artist supply stores also, such as Dick Blick.
I do recommend testing ANY marker on scrap fabric first.