I also vote for embroidery floss and to use a double square knot when tying. I have never had a problem with the floss untying/unknoting doing the double square knot. I just saw a baby quilt I made and tyed with a double square knot 12 years ago that has been passed on to many babies, washed weekly and it is still tyed also has not faded because I used good thread count fabric. The knot makes the difference whether the quilt stays tyed.
By double square knot I make a square knot and repeat it again.