K.M.
L.-
you could try using hair spray. I've had good luck with that in the past. Just spray a good amount on the pen marks and rub it in- don't let it dry- and then toss the pants in the washer and wash like normal. You may need to repeat it a couple of times, but, like I said, it has worked for me.
Hope it works-
Merry Christmas!
K.