you could make a small enough knot that you could pull it through to the back or you could not make a knot at all. maybe start with your needle at the back of the doll's head and pull your floss through to the front until it's no longer poking out the back. don't pull so hard that you pull it through. the extra floss on the inside of the doll should be enough to anchor the floss once you do the rest of the stitches.
hope this helps!