You might try a solvent and a cotton fiber filter you have to place the cotton fiber filter on the side of the marking and poor the solvent on the oppsit side you may need to press and move the fibers of the pants just a little to loosen the marker from the fabric. After you have done this go to the sink and rense with warm water and soap to remove the solvint. Do a test spot to make sure the fabric can take the solvent without discolouration of the fabric.