Ugh, not fun!! Drink a glass of wine and browse pinterest for an effective way to clean it with a more child friendly way maybe?
Did they at least apologize? I will be honest, I wouldn't expect the parent to pay to clean my carpet because it happened on my watch but I definitely would expect an apology for their child's behaviour!! To not even acknowledge it is quite rude IMO.
Here's a link I found when I did a quick google search...the comments have some helpful tips as well
http://armywifetoddlermom.blogspot.c...t-in-3_16.html