Mine are the same- black marks and mud all over!
Not tried it yet but my boss said she puts her daughters to soak in something you soak babies nappies in!!? (I pressume the towel type!?) - have no kids so not got a clue what its called!! But worth a look when shopping me thinks!!
Having brought up two incredibly dirty children I can vouch for the vanish bar soap - just scrub it on the stain with a nail brush, dont rinse and then chuck it in the washer.
It removed all the unmentionable stains from their babyhood